sfpd yanks drivers pants

i was standing on the corner of 4th and mission, around 6:00, heading down to 850 bryant for jail solidarity when this happened.

for some reason this guy was driving a 1980ish honda prelude westbound on the north sidewalk of mission street. when he approached the corner of 4th and mission, he noticed the cops and they noticed him, he gassed his car a bit in reverse, then stopped, then continued to back up slowly. 7 or 8 motorcyle cops them came around the corner towards him, along with a few foot cops on the sidewalk. at least one of the foot cops pulled his gun and told the guy to get out of the car. they then came around to the drivers side of the car and pulled him out, placed him in the gutter, and somehow pulled his pants down around his ankles.

in the photo you can see the guy laying in the gutter with his pants down, the car is behind him on the sidewalk

what on earth would motivate the cops to pants this guy?