Replay aside, I know exactly why this happened, and its happened to me before: the R is following the QB and doesn't know until he turns his head downfield whether this is IG or not -- most likely, he needs help to make that determination. By the time all that is done, he doesn't know for sure where the spot of the pass is. In this case, the result is devastating and looks absolutely ugly but its easy to see why it occurred.
Any suggestions on a mechanic that could prevent this sort of thing? When I now have a possible IG situation, I go to the spot immediately and make my wing come to me. If we determine IG, then I drop the flag.