Sorry about your accident... hope you get well fast...
I see where a few other have had the rear brake lock up... I was wondering if perhops overspray from lubing the chain got on the disk... that might cause it to grab like glue...
I dont' use the rear brake very much... those two big disks on the front really do a good job of stopping... My Harley has one front disk, weighs about 200 lbs more than my SM, and I mostly use the front brake on that as well..
Anyway... Good luck..
Mary