Looks like a little bit of rubber coming off the tires as they apply additional (and harder) braking action trying to make the taxiway.
I'm not an RJ85 pilot, but a quick search identifies the aircraft does have an anti-skid system (electric) so it is unlikely (but not impossible) to be locked brakes...plus, when that does happen the tires tend to do a little more than just give a puff of smoke...at least, the one or two times its happened to me
That is always an aircraft I've wanted to fly.