Papi's problem was he was way "weight-forward" on his stride - he lunged at the pitch. He wasn't making an attempt at the ball but couldn't keep the bat barrel back because of his forward momentum. The bat barrel did cross the plane of the plate.
Had he been able to fall away from the plate or make any movement backward, he may have got the call. All his movement was toward the pitch/pitcher, so he got boned on the call. To the ump he probably looked more like he got fooled by the pitch than he was clearly attempting to get away from it.