lol, weird al ! I think in my case, it's a lack of co-ordination and sometimes exhaustion......and sometimes a combination.
I'm extra impressed, as riding a 'two wheeler' when growing up was a rare treat. (Dad thought it to be 'unlady-like') so rides were sneaked on friends bikes. Needless to say, I never developed great riding skills. (that sounds so pathetic....rofl) Now give me a pair of roller skates........(apparently these ARE ladylike).