Because they usually got their profit from the annual pension and their territory, it was inevitable to return those favors with public duty. For that reason it often happened that nobles were pillaging the battleground.
Even if it is a bad thing to do, the kingdom hadn’t a surplus of budget to the degree of always being able to pay the nobles a salary and thus this pillaging was tolerated.
Since there was no war for more than 200 years, pillaging itself didn’t really occur. This time the opponent was a monster. Pillaging from a monster was strange in itself as well