[PSU213]

Here are my random thoughts on the play:

First, as someone mentioned before, there is a difference between accidental and incidental. Based on the description this was definitely accidental, but not necessarily incidental. (In fact, the NF rules specify an additional penalty if the interference is intentional--thereby implying that PI can be accidental.)

Rule 7-5-10a in part says that PI occurs when "Any player of A or B...interferes with an eligible opponent's oppotunity to move toward, catch, or bat the pass."

Based on that the defender would have to do something, even if it is accidental, to interfere with the receiver. In other words, just laying there and having the receiver trip over the defender is not enough for PI. IMHO, I would say that if the B player slipped on the mud and slid into the receiver, you would have DPI. If he falls, stops sliding, and the receiver trips over him, then I don't see a foul.