AFAIK, they knelt spontaneously, in order to:
1. Demonstrate their affinity toward Antifa/BLM and
2. To demonstrate their submission, to not be attacked by the rioters.
I am moved to respond, how brave of them. Someone had to be lead for that many kneelers. There look to be a couple off to the side... I take it back I thought I saw someone not kneeling, however, closer examination reveals otherwise.