Most tyre bubbles are caused when you go over a pot hole in the road too fast. The tyre gets "pinched" against the wheel rim with too much force and damages it causing the bump and thus a ruined tyre. Dealers should not replace tyres that you damaged by going over pot holes too fast.

Oh course they can be caused by a defect in the tyre in which case you should be covered, but I don't know how one goes about proving which one caused the bubble.