These pictures taken at turn 9, where Marquez crashed out and you will see this was a clear mistake from him.
Lap 8, while everything okay, he was pushing 1:38:0xx, he did not ride over the dark patch. Same as Martin.
MM:
JM:
Lap 9:
MM was all over the patch, very wide, => crash:
JM, a little bit wide, not as wide as MM, did not crash, but the bike shaked a little bit, according to "mylexicon".
I tried to find some pictures from another laps, for example, Acosta following by Bez goging at the same turn 9, lap 4. Both managed to avoid that patch, by quite a large distance.
I suspect that at turn 9, the correct racing line is at the right side of "the patch", any wider is basically a mistake, and Marquez has committed that big mistake by carrying to much speed, very wide, rode all over the patch and crashed.