I agree that he had a weak game, but on that specific play Aho saw TDA's clearing attempt got partially blocked and bounce right to McAvoy at the left point. By all indications it was about to be a shot, so Aho turned and stick-checked Pastrnak who was passing through the slot for a deflection or rebound. But McAvoy passed on the shot and carried the puck all the way over to the right point, which left both Aho and Pastrnak with their backs to the play. None of that is particularly problematic for Aho.
The blame on that play lies with DeAngelo. Not just for failing to clear the zone, but for leaving Haula completely alone in the slot in order to front a point shot which was already covered by another player. It wasn't an egregious defensive mistake, but it turned a nothing play into a wide open tip-in. Aho simply happened to be in the area with his back to the shot because he was still following Pastrnak out of the slot, but Haula was TDA's man.