It's very historical and not at all unusual. Mercenaries only fought when paid - and on time. If payment was late or not at all, they stopped fighting your enemies. At best, because from that moment they started to focus on getting paid. Being in the business of violent enforcement ('war'), they had ways to convince their paymaster rethink his sloppy payment policy.
In Dutch and in German (probably in other languages too) there still is a proverb: "Geen geld, geen Zwiters" or in English: "No money, no Swiss". The Swiss were well known as excellent and courageous soldiers, 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 it difficult to find future employment.
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.
- Geen Geld, geen Zwitsers (Dutch)