Originally Posted by TandemGeek

Yeah, here's the deal on that one....

Unlike your single bike, if you leave your handlebar unattended on a tandem while you're stopped with your stoker mounted and they shift their weight left or right of center, the tandem will lean that way and the front wheel will immediately flop over. When the front wheel flops with the stoker still seated you don't really have much of a chance of keeping the tandem from falling over underneath you since the bars (your only point of leverage) are already askew.