From my point of view there seemed to be 2 things that combined to bring Musquin down on the last lap of the race.
First, James should have gotten out of the way more and let those guys by in an easy part of the track when they came up on him and it's 2 guys fighting for the lead. The perfect place was the flat start straight..... just go slow down the start in any straight line and they blow by you and it effects no one.
Second, Marvin cased the triple he was going for outside of James being in his way and almost went down because of his own "coming up short" on that jump. Now, was James a factor and did Marvin take a different line / rut on the take off than he had been taking earlier, I don't know. If Marvin moves over 1 or 2 ruts from his normal line, there is the possibility that the launch was lower and more worn in than what he was used to and caused the "almost" crash, therefore James being there screwed him up. But those guys are adjusting to different lines the whole race when they come upon lappers (James being the 8th guy that they were lapping at that point) and should be able to do the jumps from almost any line on the track with the proper run at it.
I think they were both at fault here to some degree, and Dungey is always there to capitalize, but boy was that close to being a pile up of the leaders on the last lap, almost giving Reed the win!!!! Racing things happen, look at Hahn, Short, and Nicoletti who are all injured in a racing incident that just happened. It's part of the deal. No matter who you are, you still need a little luck on your side from time to time.