<h5>It's very historical and not at all unusual.</h5>

Mercenaries only fought when paid - and on time. If payment was late or absent, they stopped fighting your enemies. At best, that is. Because from that moment they started to focus on how to get paid. 
Being in the business of violent enforcement and premeditated murder *(we call that 'war')*, they had ways to convince their paymaster rethink his sloppy payment policy. 

In Dutch there still is a proverb: "Geen geld, geen Zwitsers" or in English: "No money, no Swiss". The Swiss were well known as excellent and courageous mercenaries, as long as they were paid on time. The moment payment was late, they went on strike. 

It did happen mercenary bands changed sides, but that was bad for business. Their reputation was very important, such a band would find future employment difficult. 

Marching in a threatening way to the capitol, or wherever their employer happend to be, was (and is) a way to get what they want. It did happen, for example Antwerp was unlucky enough to be pillaged twice in first the [Spanish][1] and later the [French][2] Furies. The French Fury failed. Those troops were not mercenaries, but regular troops. If regular army units pillaged a city over payment issues, you can be sure mercenary units wouldn't be any different. 

[Mercenaries][3] are as old or nearly as old as the 'oldest profession in the world'. My answer focuses on Swiss and contemporary mercenaries, but the first mercenaries probably used their payment for services from aforementioned oldest professionals. 

[Cretan archers][4] and [Balearic slingers][5] were famous and for hire. Even older were [Nubians][6], employed in the [Old Kingdom][7] in Egypt. 

- [Geen Geld, geen Zwitsers][8] (Dutch)

  [1]: https://en.wikipedia.org/wiki/Sack_of_Antwerp
  [2]: https://en.wikipedia.org/wiki/French_Fury
  [3]: https://www.history.com/news/6-legendary-mercenary-armies-from-history
  [4]: https://en.wikipedia.org/wiki/Cretan_archers
  [5]: https://short-history.com/balearic-slingers-993b52917f44
  [6]: https://en.wikipedia.org/wiki/Nubians
  [7]: https://en.wikipedia.org/wiki/Military_of_ancient_Egypt#:~:text=Egyptian%20soldiers%20worked%20for%20pay,Dynasties%20XIX%20and%20XX%2C%20c.
  [8]: https://nederlandseofficierenvereniging.nl/geen-geld-geen-zwitsers/
  [9]: https://www.zitate-online.de/sprichwoerter/altvaeterliche/5809/kein-geld-kein-schweizer.html