Sirius (SIRI - Get Report) has agreed to buy Pandora (P) for $3.5 billion.

"It's all about market share," said Matthew Cheslock, an equity trader with Virtu Financial. "We saw this deal coming a little while ago when Sirius put three people on Pandora's board and made an investment."

Plus, after last week's rally in stocks, Cheslock isn't surprised to see a pullback on Monday, with the Dow down some 143 points in midday trading.