how did I spontaneously combusted while riding a bike- please read.?

[keeponsmiling]

basicly me and my friend rode to my other friends house to get him and then we got him and he got the bus one stop and we rode after the bus, he got off high fived my other friend who was in front of me as his bike had no breaks and i felt a stinging in my leg i looked down and it was smoking, i hit it out i looked after afterwards ( i was wearing tracksuit bottems and jeans becuase i live in london and its freazing) and there was a huge burn hole in my tracksuit bottems less of a hole in my jeans and my skin was burn a tiny bit, it was on the outside of the bike, so im wondering how did i get on fire?
i didnt have a lit ciggerette and i passed one person and they wernt smoking.
sorry for my bad spelling and grammar