The below isn't necessarily fact, though to my understanding this is how it works CB gets first crack at INT
CB gets first crack at PD
(both of the above are effected by the CBs orientation, the CBs attributes, and can be negatively effected by the WR's attributes.
WR then gets a crack at the ball (effected by WR orientation, WR attributes, can be negatively effected by CB attributes).
If the WR catches the ball "cleanly" the CB will then attempt to hit him and get a tackle.
If the WR catches the ball but doesn't do so cleanly (as in it bobbles in his hands). Then 1 of 3 things happen...
1. The CB attempts to deflect the bobbling ball out of his hands.
2. The CB hits the WR, causing a drop that is recorded as a PD.
3. If either of the 2 above attempts fail, the WR either catches ball, or he drops the ball.
Bobbling is much more common in players with low catching. Check out the replay below of a HB with 20 catching...
http://goallineblitz.com/game/replay.pl?game_id=370093&pbp_id=14500842And then, if the WR does drop the ball (either instantly or while bobbling) the CB gets another chance at a INT/PD).