Oops. It seems the market did not at all like Internet security company Symantec's (NASDAQ:SYMC) $13.5 billion bid yesterday for storage software maker Veritas (NASDAQ:VRTS). Between the time Symantec broke the news to the markets and the close of trading on Thursday, the two companies had lost a combined market capitalization of just under $1.5 billion, with almost all of that coming straight out of Symantec's valuation.

That seems to be the consensus among market watchers, at least. CBS MarketWatch (NASDAQ:MKTW) argued in a piece last night that investors are abandoning Symantec in droves out of twin fears that (a) Symantec is significantly overpaying for Veritas, and (b) is likely to wind up with a slow-growing software behemoth for its trouble. And there's merit to that view. After all, if you value the transaction at Symantec's share price from yesterday morning ($27.38), Symantec will be paying nearly seven times Veritas' sales to acquire its target. That's considerably more than, for example, the three times price-to-sales ratio of Oracle's (NASDAQ:ORCL) finally successful bid for PeopleSoft (NASDAQ:PSFT).

But valid as that view is, it does a better job of explaining what has happened to Symantec's stock price over the first half of this week than it does of explaining yesterday's 8% drop in the value of Symantec's shares. Since Tuesday morning, when rumors of the buyout began circulating, Symantec's share price has plunged from nearly $33 to yesterday's close of just over $25 -- a 24% drop in all.

It seems to this Fool that the real reason for yesterday's final (Symantec investors hope) plunge in the company's stock price was not the confirmation that Symantec would buy Veritas. Rather, it was the news that Microsoft (NASDAQ:MSFT) was entering Symantec's market with its purchase of spam fighter Giant Company Software. The fact that security software rival McAfee (NYSE:MFE) fell even harder than did Symantec yesterday -- down close to 11% -- supports that view. When Microsoft makes goo-goo eyes at a market, it's the current suitors that get the shivers.

Keep that in mind if Veritas' shareholders refuse to approve this merger. Because even if the deal falls through based on the decline in Symantec's share price, those shares won't return to their pre-buyout offer level. Rather, they'll remain weighted down with Microsoft angst. (And yeah, verily, Veritas' shareholders could decide to turn Symantec down now. Because at Symantec's current share price, its offer of 1.1242 Symantec shares per Veritas share is worth just $28.25 per Veritas share. That's less than a 1% premium to Veritas' current share price, and probably not worth the bother.)

Can't get enough of merger news? Read Tim Beyers' marathon coverage of the Oracle-PeopleSoft merger in:

Fool contributor Rich Smith has no position in any company mentioned in this article.