I don't think that definition helps much[1].

For one, arguing that the cause is usually central bank inflation would eliminate a great deal, including the dutch tulip bubble.

What about bubbles that took place before central banks existed or were powerful?

What about bubbles that had nothing to do with inflation like the tech bubble?

Must other markets remain stagnant to satisfy this definition of bubble? What if one market goes up a little, and another goes up a lot?

If there is an underlying cause, how can the movement be "for reasons unknown"?

Can bubble tendencies occur in only one direction? Is it not possible to have a bubble in negativity towards something?

[1] I may be an armchair economist, but my armchair is located at a large academic institution.