[BoomerSooner]

By calling it "incidental" I think the OP may be suggesting the contact was "unintentional", and it most likely was unintentional from the description of the play. The key, however, is that intent is not a factor when determining interference on a fielder making the initial play on a ball.

The runner/batter runner has a responsibility to avoid interference with the fielder in this situation. The out isn't for the contact, but rather for failing to avoid interfering with the fielder. Had the fielder still caught the ball, there wouldn't be interference even if the contact was intentional because the contact didn't interfere with the fielders ability to make a play (although there might be other penalties depending on the nature of the contact).