A few things to consider:
If Callahan hated the Raiders so much why did he accept the HC job? Why didn't he follow Gruden to TB?
Hate them now? Yes. Hated them then? Hard to believe.
No one would seriously jeopardize their career by intentionally sabatoging the team, just before the SB! Again, hard to believe.
If the players believed Callahan sabotaged the team, wouldn't Al Davis believe it too? Because if he did believe it, we know he would have immediately fired him.
Which leads me to a more plausible explanation: I heard Warren Sapp say on a NFL Channel show a couple of years ago his explaination for some of the problems the raiders have. He said it was common for the defensive coaches to have a game plan in effect and they would practice it all week, then on Friday, Al Davis would look at the game plan and force the coaches to completely change it to more his liking.
He said that happened every week. It may also explain why Fox walked out on the raiders many years ago. Al Davis was an egomaniacal meddler. If anyone was capable of sabotaging the team, it was him. That, I would believe.