Most of the times they do that to impress the ladies/and young college girls to be exact they love that stuff (as long as you don't make love to the tarmac), trust me this guys get laid big time.
There is always a relation between loud music and our moods, some react differently to it and some get hyper. Ever noticed how matatu drivers drive fast especially when playing songs with catchy beats especially when the woofer is on. Especially mat za buru, umo, kayole. (by the way kitambo mathrees had a switch for the woofer hidden somewhere, either the AC button ama hapo chini ya steering do they still do it ? )