Sedrick Irvin is a quality running back, and it would be foolish to turn one away who was already committed.
He's already polished in the passing game, and I believe this would certainly have made him a good candidate for significant playing time as a true freshman.
I find it extremely unlikely that the coaches would ask him to decommit, especially since Limar hadn't announced just yet. In this day and age, it's not wise to consider a "silent commit" as a hard committed player, given how we got badly burnt in the past.
Now, if someone said that the coaches had told him they were looking at taking another running back or two in this class, and that they were both high caliber backs, then that's entirely possible that this could have made him reconsider his commitment. That happens all of the time, especially at high profile positions.
I suspect Irvin simply wants a higher chance of getting real playing time, and have a higher chance of being "the man." If there's an easier route for doing so, such as Michigan State, etc., then more power to him. I don't begrudge him for making such a choice.