Could be numerous factors in this. I've only heard of an aircraft being returned to stand due to a tech fault which has made itself known during the taxi or if, after doors have closed, something has flagged up at the boarding gate or indeed on the taxi which requires a pax to come off etc.
If she was an UnMin then fair enough. Perhaps, due to whatever reason this young woman's bag was going to be going without her (Failure of ground crew to communicate she was not on board, system error, card reconciled in error etc) and it was quicker to get her on than to look for and get her bag off. Flying her bag without her is obviously a no/no. Perhaps the captain had to return to stand for an unrelated matter and agreed to take her on anyway if she had no checked bags.
Could be one of many reasons.