Jackson really wasn't fired for on-court reasons (although there's plenty to criticize there), but because he sucked at dealing with management.
A few things I've read, which aren't necessarily substantiated:
The FO gave Jackson a lot of room to coach. They asked him to bring in a new X and O guy after Mike Malone left, but he refused
Speculation: he was trying to push Erman out? I mean, Erman got so paranoid he started recording conversations.