On Saturday, the driver of a pickup truck was blocked on the road by Black Lives Matter protestors. After being stopped, he slowly drove through them until they got out of the way.
This angered the BLM protestors, who shouted at the police.
Police were reportedly inside the Hy-Vee due to the demonstration. It seems the demonstrators wanted the police to arrest the driver of the truck, who was driving on a public road, and decided to continue driving on that road despite people blocking it.
You can watch the subsequent reaction here (be forewarned it includes some expletives).