Both Shanahans are definitely a part of it, I agree. I think RG3's having real mental trouble adjusting to being a real pocket passer, though. I pointed out to some Redskins fans (before I got banned.....) on their forum that half of RG3's passing yards came from shovels and screens to Alfred Morris and the receivers, and that I wasn't convinced RG3's a good pocket passer yet. I still have yet to see it, but I'm not going to pretend that I'm either an expert or someone who has closely studied all of his tape.
I remember the video you're referring to. Someone posted Wilson highlights in that thread by comparison, and the difference in running styles was striking, to say the least. RG3 would rather dive and take a huge hit to gain another yard than save his body last year. Kind of ironic that he ended up tearing his ACL while planting to throw without being touched on his craptastic stadium.