Some biased, maybe inaccurate, context...
I don't think Masters is exactly respected in track circles. Being of similar age and ability, I've had a general knowledge of him and his behavior since high school. Again - I can't stress enough that I've never talked to him, could be a really nice guy...but stories I've heard, races I've watched all point to someone who is somewhat self-centered and not level-headed. I don't think I'm the only one who has this opinion. So when Master goes down over McGregor, I think his reaction is akin to Susie Favor Hamilton taking a dive, might be accidental but there is history there. Also consider Master's DQ for impeding Reed Connor in 2012 1500m semis.
I've watched the race several times and I don't know how you could say you were running the race of your life. It went out in 2:12, half this site would have been "having the race of their life". With 100 to go Masters was clearly at the back of the pack. No way he finishes top 3 like he dreamed of. To his credit, when Masters fell he popped up and tried to finish--no harm, no foul. McGregor took a much harder fall and was lucky not to've broken a bone.
Was no appeal fouled by Texas? McGregor appeared to've been contacted by Perkins, which caused the fall? I noticed Perkins bulling through a couple of runners in his semi-final heat as well. Some contact may be inevitable in a race like this, but there is the beginning of a pattern of aggressive, physical racing tactics with Perkins.