I don't know the whole story. He had some injuries last year, one to his neck I believe. I think that actually scared him enough that he decided to retire. That's what I've heard anyway. I heard he was "ok" enough to play last year after the injury, but decided he didn't want to, and wanted to walk away from the game healthy. The Cowboys (either Parcells or Jones) decided to not make a big deal out of it, and let him make his own decisions. I think he's got a young kid (or two now) and wanted to devote his time to his family, too.
I think he was a great player! I watched him play for four years as an Aggie, and then in the NFL. He's the leading tackler in Aggie history (and until recently we actually had good defenses). He was a true overachiever, and I think a great role model. He's also from down here in south Texas (Rockport - little fishing community on the coast, where you can buy shrimp off the boats), and is beloved in the area.