The whole set of wheelchair contestants were at fault. The are coming up at a speed of about 20mph, the come in on the left side between the aid station and the runners. Now, the wheelchair athletes that cause the actual collision were a bit further back, as was the aid station itself when the first in the line race through on the left. However, there was (almost certainly) an indication that the aid station was coming up and was on the left side.
One problem with having the wheelchair athletes leaving before the women start and them being ahead of the men, in this case by an hour, is that the latter part of the course is active for an even longer period of time.
Still, placing them either on a different part of the course (on the other side of barriers, etc., which might lead to different distances (along the same general route) and hence a slightly altered start and/or finish.
Also, remember that the top women are not at all used (like, it virtually NEVER happens to these athletes) to being past by someone that they were not running with in the last 5, 10, 20 seconds