What did he do? Well ... the NCAA heard that he was talking with Deion Sanders, and they were investigating to make sure that there wasn't anything going on contrary to NCAA regulations regarding outside contacts, agents, and so on. When asked, Bryant said that he never talked to Sanders.
It turns out (a) Bryant did, in fact, talk to Sanders, and (b) there was nothing wrong with their conversation. But lying to an NCAA investigator is (obviously) against NCAA regulations, and so lying about what turned out to be an innocent contact got him in trouble.
I'm going to have to start making a list. Because this keeps happening over and over again.
Al Capone. John Ehrlichman. Martha Stewart. Kwame Kilpatrick. And now Dez Bryant.
It's not what you do that gets you into trouble. It's lying about it when confronted.