Rumour has it that "Johnny Foreigner" runs his vans and HGV's on cheap, badly worn or even remould tyres until they expire.
(Not convinced about the remould argument - those in the business can comment better)
The tyres then give up the ghost and get left on the side of the road by councils who don't have the money to repair potholes, let alone "sweep, vacuum and polish."
Mess "probably" accumulates over the winter, "maybe" getting tidied up when it stops raining all the time, and when the days get longer.
I hate blaming stuff on "Johnny Foreigner," but it's a case I've heard a few times. Usually after mutterings about those 3.5 ton curtainsider / tautliner trucks.