Pete Carrol is leaving USC behind this year to coach the Seattle Seahawks.  Will he want to bring some of his players with him?  The obvious answer would be that he would be more likely to draft USC players because he's already familiar with them and knows what to expect out of them.  For example he may have more insight than anyone else as to whether Taylor Mays' lack of interceptions was because of their defensive scheme or what expectations of Mays were emphasized.  He may also be planning on installing a similar offensive or defensive system and want to draft players already familiar with the playbook/terminology.  This insider information could also cause him to take certain players from USC off his board entirely, but the draft is about which team likes a player the most, not the least, so more information and familiarity with players from a certain team; if that were the only factor, would lead to more players from that team being selected.

There may be other factors to consider though.  Carrol may set himself up to look pretty bad if he drafts a player from USC and that player busts in the NFL.  A coaches job can often hinge on perception (mostly when they're not winning).  Picking busts, especially in the first round, is a surefire way to doom a new head coach.  When you ad into this a convenient narrative for the media to latch onto as a story: "Bad Coach brings bad player from his former school to ruin team", it may be too much of a risk for Carrol, especially in the first round.

I'd look for Carrol to grab a couple of his former players in later rounds and to give several un-drafted free agents from USC a shot, but I think he'll steer clear of USC players in the first round because the "image" risk is just too high.