Not enough context with that video. Try this:
Kareiva of Belarus is on the inside lane. She almost goes down, and in the process she pushes Russian Tomashova out. Tomashova's changed line actually causes her to make contact with Uceny (see 1:07--arms touch), but the bigger problem is that Tomashova forces Aregawi out, and Aregawi hits Uceny.
The original move that caused Kareiva to stumble? I think it was Aregawi. She aggressively splits two runners (see 0:23) and then tries to squeeze in at the start of the curve. Aregawi then clips Tomashova (1:18) whose back-kick hits Kareiva.