check the nut that holds the bearing in place,if this has not been punched down after bearing fitted then they can easily slacken off,other obvious ones is bearing being fucked or wheels not tighten fully

dan,this happened to my mate when he ran around with a noisy bearing,it puts all the load on the shitty 30mm nut that holds the bearing in place,he took a right hander and it decided to pop out busting his ball joint in the process