The Commonwealth Bank of Australia (ASX: CBA) share price has been outperforming the market consistently for the past 18 months.

But that outperformance came to an abrupt end in July, with the banking giant's shares taking a tumble.

Over the course of the month, the CBA share price lost 3.7% of its value.

As a comparison, the S&P/ASX 200 Index (ASX: XJO) rose by a solid 2.35% during the period.

Why did the CBA share price underperform?

Investors were hitting the sell button last month after the great rotation out of banking stocks into mining and healthcare stocks appeared to begin.

For example, the BHP Group Ltd (ASX: BHP) share price gained almost 7% last month, and the CSL Ltd (ASX: CSL) share price performed even better with a gain of approximately 13%.

In addition, it is worth noting that a number of brokers reaffirmed their sell ratings on Australia's largest bank ahead of its full year results release later this month.

One of those was Macquarie Group Ltd (ASX: MQG), which remains bearish on the bank.

What did it say?

According to the note, the broker has an underperform rating and lowly $105.00 price target on its shares.

Based on the current CBA share price of $175.06, this implies potential downside of approximately 40% for investors over the next 12 months.

Macquarie warned that CBA and the big four banks could soon lose the buying support from big investors that has propped up their share prices. It said:

Looking forward we see several key risks to these positioning trends which have supported banks including; (1) super funds turning negative on the bank sector, (2) a return of unlisted activity and increased preference for offshore investments seeing allocation to Australian equities fall, (3) US regulatory easing unlocking billions in capital returns, seeing global financial investors fund US bank buying with Australian banks, where capital returns have already played out (see Closer to the bottom of the barrel). This combined with downside risk to FY25E/FY26E earnings (see Inching closer to the edge) should drive banks underperform.

In addition, the team at Bell Potter revealed that it was underweight CBA due to valuation reasons. It commented:

While acknowledging CBA is the highest quality bank in Australia, the significant underweight position is driven by its excessive valuation, which sees it trading at a material premium to both its peers and its own long-term history.

In light of the above, it wouldn't be too surprising if the CBA share price continued to underperform over the next 12 months.