Did you find out what this was? I did a quick google but couldn't find any news about it but I wonder if it was an instance of -
en.m.wikipedia.org/wiki/Critical_Mass_(cycling)
a so called "critical mass" cycling event. Basically a protest by cyclists about cycling conditions in cities. Cycling - since Bradley Wiggins - has become a human right protected in law.
Surrey is scarred at weekends when you go out for a nice drive by rows of lycra-clad buttocks pedalling away 7 wide across the road.
I feel like protesting by driving straight through them....