He was asked for his texts to the deflators, which would be screened by his lawyer. He refused. They asked for a follow up interview with the deflator after they gained access to his text messages, and the Patriots' lawyers refused to allow it. The deflator denied even taking the footballs to the bathroom. When they showed him the security footage, he changed his story a few times.
The decision was severe because
1) cheating
2) obstructing the investigation / lying
3) previous history with cheating
A lot of the Patriots fans sound like Bruins fans when they're confronted with all of their garbage like diving, hypocrisy, head hunting, etc. On the one hand they claim the PSI of the football is irrelevant and then they bring up that they scored more points in the 2nd half with footballs in the proper inflation range.