There is some better footage from the crash on Sporza, and given the race interview by Benoot, it was 'simply' an overlap of wheels.
The thing I struggle with is: Benoot saw riders (EF Education) coming on his left, noticed Tim Van Dijcke dropped the pace a bit, so he went left of Tim Van Dijcke to guide WvA, but when upping the pace, he stood on the pedals. I know he doesn't have a strong acceleration in the saddle, but why oh why did Benoot stand on the pedals, while abruptly veering to the left? WvA was overlapping yes, but he had no chance by how sudden Benoot moved over. I can see why Benoot feels bad about it.
big edit: after seeing the replay several times more, Benoot was already quite a bit to WvA's left. I don't know why WvA lost balance, most likely he was following Benoot and came into contact with Kirsh? Because I don't see the overlapping wheels scenario holding now.