While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Wells Fargo downgraded Broadcom (NASDAQ: BRCM) from market perform to underperform on Monday after the chip giant said it was looking to exit its cellular baseband business.

So what: Along with the downgrade, analyst David Wong reiterated his price target of $27-$29, representing about 20% worth of downside to the current stock price. So while momentum traders might be attracted to today's double-digit pop, Wong's call could reflect a sense on Wall Street that the rally isn't fully justified.

Now what: Wells also lowered its 2014 earnings-per-share estimate for Broadcom from $1.47 to $1.45 and its 2015 view from $1.83 to $1.72, on lower revenue assumptions. "While we had thought that it was questionable whether Broadcom would succeed in establishing a strong position in the 4G baseband market, Broadcom's announcement that it is looking at exiting this business does have a material impact on our view of the company and our estimates because it confirms what was previously a concern that might not have materialized," said Wong. With Broadcom shares now flirting with their 52-week high and trading at a clear PEG premium to the industry, it's tough to disagree with Wells' caution.