In the first part of The Deed of Paksenarrion (Sheepfarmer's Daughter) by Elizabeth Moon, the main character is part of a force garrisoning a fort at a place called Dwarfwatch. This fort is then essentially sieged twice, first by the Halverics Company to whom they surrender, and again shortly after that by the forces of Siniava.
The reason for the second siege is reasonably clear, given that Siniava wants to somehow conquer all of Aarenis. However, after finishing the books, I cannot recall any explanation why the Halverics attacked Dwarfwatch.
Given that they are a mercenary company, they were obviously hired by someone to do so. Is it ever explained who their contract was with?