Happened to me a year or two ago - inch long stainless steel wires ion the grill. Inspected the brush, and it was old and should have been replaced much sooner. The synthetic black material which housed the bristles had been compromised over the months by too much heat, causing bristles to come loose.
Take home lesson for me: don't expose the brush to excessive heat, and replace them more frequently. I've not had a problem since, and I carefully inspect the grill after each brushing.