Why is everything called a "bubble"? A bubble by definition is something that is unnatural, unjustified, and rare; not something something that occurs regularly.
My understanding is that the "bubble" metaphor refers to the price "inflating" by an extraordinary amount, pushed by nothing but air, and then suddenly collapsing. I've always seen it used (outside the trading community) in pejorative sense, and never noticed any implication about the price after the bubble being higher or lower than before.