I cannot say I am overly surprised at this development.
Castille was a highly-touted recruit, and he was very impressive in his first two years at Alabama. He was a bit smaller than most traditional fullbacks, but he was very versatile, and we used him to a degree like LSU used Jacob Hester the past couple of years. In both 2003 and 2004, he was easily one of our best offensive players. In 2003 he had a couple of rushing touchdowns and over 20 catches while starting as a true freshman, and in 2004 he racked up almost 400 all-purpose yards to go along with six touchdowns despite only being healthy for half of the season.
His career, though, effectively ended in 2004 with the major knee injury he suffered against Tennessee. It was a pretty nasty ACL plus injury, and the truth of the matter was that he should have redshirted the following year, but he didn't, and he clearly didn't play anywhere near as well. By 2006, Shula had decided to put him at tailback in the vaunted Jumbo package, and he rarely saw the field aside from that. I've long since contended the biggest problem Castille had at Alabama was poor coaching.
Now that is in the NFL, with the knee fully rehabilitated, and someone is using him as he probably should have been used all along -- a lead-blocking fullback who gets the ball a fair amount in his own right -- I'm not surprised that he finds himself starting.