United receiving many penalties before was a testament to their fast-paced, counter-attacking style as they used to often have opposition defenders isolated against pacey players (Nani, Ronaldo, Valencia, Young, Welbeck etc) running at them with the defenders running backwards towards goal (and the chances of the defender having to resort to fouling or making a mistake are much more likely in such a situation). Whereas in comparison, the other teams in the league at the time would not have had as much pace, would not have had the quality to have used their pace in such a dangerous manner or would have had a slower and more pragmatic build-up which stops at the oppositions box. Back then United used to dominate teams more (especially at home) and as a result would have spent a lot of time in the opposition box than they do now either passing the ball or attempting to take players on and as a result would have forced more errors from the opposition defenders (foul, handball etc) which would have resulted in more penalties too. Once United's play improves and they become more consistent and more dangerous going forward (Di Maria back from injury and in full flight will help in this regard immensely) then they will start getting more penalties.