Auburn always takes one or two people in each class that many consider sleepers. A lot of time those are late in the class. In the case of Marquis McClain, the Tigers accepted his commitment in late July. McClain was a 2* at the time (he was bumped to a 3* shortly afterwards) and wasn't even on the radar of some of the recruiting sites.
Mississippi State pushed for an official late in the cycle and hoped they could steal him away. He never went on any visits after committing to Auburn, though.
McClain is the lowest ranked of any of the wide-receiver commits on the board, but I still expect he'll be making contributions to the team quicker than many may think.