After 30-ish years on lugged-steel Trek, I added a seatpost-style water bottle rack (2 bottles) to my handlebar stem. When the bottles are full,or nearly so, I have experienced a wobble at high speeds that was not present before. Recently it occurred on a fast descent in a group and I thought I was going to buy the farm. Finally slowed enough to get control. I have found that clamping knees against the top tube will greatly diminish it, a sort of damping effect. From an engineering perspective, this is a complex dynamics problem worthy of a dissertation by a sharp grad student. Lots of factors at play.