If that happens on a counter attack, the majority of times it will be a foul and a yellow card.
The defender tries to run across to get to the ball initially then bends his run to get goal side of the attacker, clipping his heels as he does so.
It's contact in the box, the player is prevented from getting to the ball because of that contact and if Vince Lia did it we would be getting stuck into him for being careless/old/useless/etc with etc being the worst.
It's an unfortunate pen for the defender, and maybe unlucky, but for the same reason that players put their arms behind their backs when defending- they don't want to risk ball hitting arm being given and I think a similar principle applies here. Contact in the box is a risk, so take extra bloody care not to do it.
It's not like the dives from Brown or Coveny. We're talking contact, somewhat carelessly from the defender, who runs behind the attacker, and this prevents the attacker getting the ball in the box.