In the 0.4.4 version, my understanding is the QB's progression is from short to long (whether this means route length or time on pattern, I'm not sure)...regardless, since the RB is typically running a flair out of the backfield, he usually fits the criteria of both being the shortest route down field and the shortest time-wise to reach the end of the "route" and therefore be open to receiving a pass.
The net effect is a lot of passes going to RB's.