Bank stocks on Wall Street suffered their biggest one-day drop since February on Wednesday, with the KBW Bank Index closing 2.9% lower and widening its weekly loss to 5%.

The sell-off this week is the result of multiple pressures stacking up at once. A cautious earnings outlook from Bank of America Corp (NYSE: BAC) 's CEO directly sparked worries about deteriorating fundamentals in the banking sector, but the deeper cause points to persistently stubborn inflation, unusual shifts in the interest rate structure, and growing uncertainty in global debt markets.

The direct trigger: BofA CEO's cautious guidance dragged down the entire sector

The immediate catalyst for this week's bank stock sell-off came from remarks made by Bank of America CEO Brian Moynihan at the Barclays industry conference. Moynihan said the bank's third-quarter trading revenue is expected to be "roughly flat" compared with the same period last year, a projection that stands in sharp contrast to the strong trading performance Wall Street saw in the first half of the year. He also guided third-quarter investment banking fee revenue to approximately $1.6 billion to $1.8 billion, while analysts had previously expected close to $2 billion.

Following those comments, Bank of America shares plunged as much as 6% intraday on Monday, the biggest intraday drop since April last year, before closing down 5.14%. Other major Wall Street banks including Goldman Sachs Group Inc (NYSE: GS) and Morgan Stanley (NYSE: MS) also came under pressure.

Notably, Moynihan pointed out that uncertainty in the interest rate environment is currently the key factor weighing on capital markets activity. Companies are only more willing to make bond issuance decisions when they have greater certainty about their future financing costs. In other words, the problem is not just the level of interest rates - the violent swings in rates are equally suppressing corporate financing appetite and banks' capital markets businesses.

The deeper root cause: inflation stays hot and banks' net interest margins get squeezed from both sides

The reason BofA's earnings warning triggered such a strong market reaction is that it exposed a deeper issue: inflation that keeps coming in above expectations is pushing interest rates higher in a way that hurts banks. The source of this inflation round can be traced back to roughly seven months ago when the US-Iran conflict erupted, causing a global energy price shock. Oil prices surged and quickly transmitted into broader price levels. The Fed's preferred inflation gauge - the Personal Consumption Expenditures (PCE) price index - jumped from below 3% in February to above 4% by May, while core PCE (excluding food and energy) also peaked at 3.5%. Though it has eased somewhat since then, headline PCE remains above 3.5% and core PCE is around 3.3% - both far above the Fed's 2% target.

The reason inflation puts pressure on bank stocks comes down to how it distorts the interest rate structure. Fixed income investors view inflation as a loss of future purchasing power of their money, so when inflation expectations rise, they demand higher returns as compensation. This pushes up long-end yields - the 10-year Treasury yield breaking above 5% this week is a direct reflection of rising inflation expectations. In the view of Seeking Alpha contributor Jeremy LaKosh, the core issue lies in banks' business model. Banks borrow in the short-term market and lend in the long-term market, profiting from the spread between the two - known as the net interest margin. In a normal rate hiking cycle, rising short-term rates push up lending rates and banks' net interest margins typically expand. But this cycle is completely different.

The US Treasury is currently trying to push down long-end yields through "selling short-dated bonds and buying long-dated bonds" operations. Data shows that since the last Fed meeting, long-end Treasury yields have risen about 15 basis points, while yields on 2-year to 5-year Treasuries have risen over 40 basis points. Although the Treasury's operations have achieved some results, it still needs to issue short-term bonds to raise funds, which further drives up short-end rates. This means banks' short-term funding costs are rising rapidly, but long-term lending yields are being suppressed by the Treasury's market operations. Borrowing costs are rising fast while lending returns rise slowly, compressing net interest margins significantly and ultimately weighing on bank profitability.

Bank stocks still face pressure: rate hikes can't ease inflation worries and rate volatility is the core variable

The Fed delivered its widely expected 25-basis-point rate hike on Wednesday, bringing the target range to 3.75%-4.00% - the first increase since July 2023 - and signaled that further hikes could come this year. But for bank stocks, the key issue is not the hike itself; it's that investors worry the rate increase won't effectively bring down inflation expectations, and instead would combine with the Treasury's bond issuance operations to squeeze net interest margins further.

Many market participants believe the current inflation is primarily driven by supply-side factors, which rate hikes can't effectively address. LaKosh thinks this view holds up in theory, but the key to judging whether current inflation is a short-term phenomenon lies in whether services inflation gets "ignited." Most inflationary pressures currently remain concentrated in the goods sector. The problem is that if inflation starts spreading to the services sector, it could require much more aggressive rate hikes to bring prices back under control. The post-pandemic experience has already proven this point: once services inflation takes root, the policy cost rises significantly.

Before the Fed's move, some argued for hiking earlier to prevent inflation from taking hold in services and to avoid further deterioration in rate volatility. Now the Fed has chosen to hike and signaled more could follow this year. But this week's market reaction shows that the rate hike itself hasn't dispelled inflation concerns. Most economists estimate it could take until at least 2028 to get back to the 2% inflation target.

For bank stocks, short-term earnings volatility is certainly worth watching, but the more important variable is whether the interest rate environment can stabilize soon. Moynihan echoed a similar view: "Rates will eventually stabilize, and I think that will help some of the trading activity." From an investment logic perspective, the impact of rising rates on bank profitability is not a one-way positive. In periods of strong fundamentals, credit expansion, and net interest margins widening in tandem, bank stocks tend to trend higher with volatility. But if the fundamental outlook is under pressure, rate hikes could instead trigger a "double whammy" of earnings misses and valuation compression. The current market reaction shows investors are reassessing banks' earnings prospects in an inflationary environment - and this repricing may not be finished yet.