The Commonwealth Bank of Australia (ASX: CBA) share price was on track for another strong performance last month.
Right up until 23 September.
On 30 August, shares in the S&P/ASX 200 Index (ASX: XJO) bank stock closed trading for $139.50.
On Friday, 20 September, shares in Australia's biggest bank closed at $144.50, marking a new all-time closing high.
At that stage, CommBank shares were up 3.6% over the first three weeks of September.
But when trade resumed on Monday 23 September, the CBA share price closed down 1.4%. And it finished in the red for the next four trading days as well.
When the closing bell sounded on 30 September, shares were changing hands for $135.39, down 2.9% for the month.
That saw CommBank stock lag the 2.2% gains posted by the ASX 200 over the month just past.
Still, shares in the ASX 200 bank remain up an impressive 34% over the past 12 months. And that's not including the $4.65 in fully franked dividends CBA delivered over the full year, after paying out the final dividend of $2.50 on 27 September.
Here's what happened with the big four bank over the past month.
Image source: Getty Images
What moved the CBA share price in September?
There was no price-sensitive news from the bank last month, but the CBA share price moved higher over the first three weeks amid ongoing positive investor sentiment.
Some of that optimism seems to have been related to CBA's strong investments in artificial intelligence (AI).
On 17 September, management announced the business had taken a "monumental step forward" in AI adoption after activating its AI Factory in collaboration with Amazon Web Services (AWS), a subsidiary of Amazon.com, Inc. (NASDAQ: AMZN).
Commenting on the AI factory at the time, chief data and analytics officer Andrew McMullan said, "The technology is advancing quickly, and we need the right infrastructure in place to support our strategic goal of building tomorrow's bank today for the benefit of our customers."
But as mentioned up top, the CBA share price took a downward turn the next week, on 23 September.
To be fair, it wasn't just CommBank stock that came under pressure, with all the big four bank stocks selling off towards the end of the month.
That came after China announced significant new stimulus measures to boost its sluggish economic growth. Those measures include a 0.50% cut to interest rates on existing mortgages, with the People's Bank of China also cutting banks' reserve ratio requirements by 0.50%.
Now, that's not inherently bad for Aussie banks.
But with commodities like copper and iron ore soaring on the news, the CBA share price looks to have taken a hit as investors rotated out of ASX bank stocks and into the big Aussie miners.
