Sometimes there is no reason. Some manager gets up for a pee in the middle of the night and phones the on-call guy to "check the site is up" or "can you re-run the report for me" (I'm not even kidding). That company saw the engineers refuse to do any on-call 'til we got new contracts stipulating on-call was ONLY for site outages, that said outages had to be verified by a human before calling (so no hair trigger automated alerting) and in some circumstances we would be paid quad-time.

But even so things were pretty pathological there. There were those of us who understood that if the site was down, we weren't making any money, and none of us would get paid. Then there were others, who understood that there were people in the first group, and they could just... not bother. And there was insufficient differentiation between the two come bonus time...

I am coming across as being bitter here, far more so than I actually am, but the OP deserves to know, it can be bad.