Somebody sharing his computer could do that.
For instance, If I log onto Yahoo!, open up another window, close the Yahoo! window and leave another window open at website 'A', somebody could visit Yahoo! from that computer and still be logged in as the previous user. PLayers could be added and dropped that way.
It might not be him specifically, but it could certainly be someone with access to his computer.
As commish, you have to evaluate all possibilities and make a judgement that is in the best interest in the league. Accidents can happen. Once. I'd lean towards cancelling the trade just to satisfy him. If another incident pops up involving him, he is S.O.L.
...and if it was a grossly unfair trade to begin with, it shouldn't be allowed anyways.
Without knowing the players involved, it's kind of tought o judge his intent. If he offerred you Peyton Manning for Kyle Orton, it's a bogus offer to begin with and there really should be only one solution, declining the offer.