Commonwealth Bank of Australia (ASX: CBA) shares had a difficult month in May.
The banking giant's shares ended April at $173.66 and started the month strongly, climbing as high as $179.23.
But the mood changed quickly. CBA shares tumbled to a monthly low of $153.67 before recovering some ground and finishing May at $165.02.
That means the stock ended the month down approximately 5% from where it started.
However, it is worth noting that the move from its monthly high to low was far more dramatic, highlighting just how quickly sentiment turned against Australia's largest bank.
So, what happened?
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Quarterly update disappoints
The main catalyst for the weakness was CBA's third-quarter update.
For the three months ended 31 March, the bank reported operating income that was flat on the first-half quarterly average. Net interest income rose 1%, helped by lending and deposit volume growth, higher deposit margins, and earnings on the replicating portfolio.
However, this was offset by lower other operating income, cash rate lag, competition in home and business lending, a weaker New Zealand dollar, and two fewer days in the quarter.
Expenses also moved higher, rising 1% excluding restructuring and notable items. This was driven largely by cloud computing volumes, software licensing, and investment in artificial intelligence capabilities.
On the bottom line, CBA reported unaudited statutory net profit after tax of approximately $2.6 billion and unaudited cash net profit after tax of approximately $2.7 billion.
Cash profit was down 1% on the first-half quarterly average, though it was up 4% on the prior corresponding quarter.
For a share trading on a premium valuation, that was not enough to keep investors happy.
Provisions catch the market's eye
Another area investors focused on was provisioning.
CBA's loan impairment expense was $316 million for the quarter. The bank also increased the forward-looking component of collective provisions by $200 million to reflect heightened geopolitical and macroeconomic risks.
Management stressed that underlying portfolio credit quality remains sound and that actual losses are still low.
Even so, consumer arrears increased modestly during the quarter, while corporate troublesome and non-performing exposures also moved higher.
In an uncertain economic environment, it seems that investors are watching these numbers closely.
Federal Budget adds another headwind
Sentiment was also affected late in the month by the release of the Federal Budget.
Some of the measures in the Budget have been interpreted negatively for CBA, adding to concerns that the outlook for the banking sector may be getting tougher.
This comes at a time when investors are already questioning whether the major banks can justify their elevated valuations. Competition in lending remains intense, margins are under pressure, and any signs of rising credit stress can quickly weigh on confidence.
Foolish takeaway
CBA remains one of the highest-quality shares on the ASX. It has a huge customer base, strong capital and liquidity settings, and a dominant retail banking franchise.
But May showed that even the market's favourite bank is not immune from disappointment.
A quarterly update that fell short of expectations, higher provisioning, and Budget-related concerns combined to push CBA shares lower.
The stock recovered from its monthly low, but investors may remain cautious until there is greater confidence around margins, credit quality, and the broader economic outlook.
