I think the brushes have to be a bit worn for the bristles to start coming out, the usualy places are where theres cracks in kerb stones, drain covers, splits in the road etc anywhere where theres going to be a good chance they will be ripped out, the other areas where they seem to fall/pull out are corners especially concave ones as the brushes will be overloaded being pushed into them, rough surfaces like cobbles etc,
You could see if it comes from a yard locally and simply ask them if they would save you the old ones when they get replaced, there may also be a old machine there that the new one has replaced with bristles scattered around it
