Why did 49ers' guard Brandon Thomas just disappear?

Daily 49ers blog for NBC Bay Area website: Going into the 2015 season, the 49ers had high hopes for guard Brandon Thomas, a former Clemson star. Yet he never played a down. What happened? http://www.nbcbayarea.com/news/sports/Thomas-Was-49ers-Mystery-Man-in-2015-368845481.html