Random seasons unmonitored?

I'm currently going through my entire library to upgrade to 265 where possible, and I've noticed seemingly random seasons from random shows being unmonitored. And some entire shows, for that matter. Any idea why it's happened? It's definitely not something I did manually - one show had seasons 1, 2, 3 and 5 unmonitored, but 4 was still normal. Ultimately not that big a deal as I'm just fixing it as I go, but I'd like to know what caused it if possible since it'll prevent those seasons being automatically managed and upgraded if available.