many years ago I was mooned twice, once on the bike, once on a bus. On the bike I was coming home from work down Gulson Rd Coventry, the lad decided to step of the pavement turn his back and drop his trousers, then he panicked because he suddenly realised I was travelling faster than he thought and I wasn't going to go round him and he scrambled back on the pavement. On the bus me and the misses were coming back from visiting friends late at night and the bus was going down Far Gosford St when a lad did the same thing to the Bus driver, the bus driver didn't slow down or try and go round him and the result was the same with the lad scrambling onto the pavement. lots of pubs in that area so I presume drink played a part in both incidents.
