What is a word that means something that causes an imbalance?

One of the most common expressions is deal breaker, meaning something that disturbs or upsets.

In what context? An upset, or a disruption, could work.  Anything else requires a better idea of the nature of the disruption (the upset) of the balance; many are specific to the way that the balance is destroyed rather than simply that the balance was lost..

Vertigo.