Sometimes the problem is inertia.

Bottles mounted on down tubes are subject to ejection if the bike slows suddenly. I'm not talking about braking here, but the horizontal shock of hitting a pothole at speed. There's a steep bumpy hill I descend fairly often. I have to make sure the D/T bottle is no more than half full, otherwise it gets ejected every time.

Mtn bikers see this often, and some even eject vertical bottle, though that's much harder.