Maybe they got out of food. Recenty I was observing a Vlandian army of 1.5k sieging my town. I had 1k army and watching near the town how the army started to starve. After 500 troops died in hunger they retreated. They tried to get to their nearest town and I followed them until the army had only 800 troops and then attacked.
Pritty amazing experience
Pritty amazing experience