Why does it let me muck sometimes and not others?

I play online at Pokerstars. Sometimes after a hand I get a chance to muck and others times not. You would think I would notice a pattern and be able to figure out when and why and lets me sometimes and not others. I know there has to be some logic behind it but I'm missing it??

There are several factors that determine whether you auto-show, auto-muck, or are prompted. Whether you're the better or caller on the river, whether you have the auto-muck setting enabled, the number of people going to showdown, whether there are side pots and thus multiple showdowns, etc.

If you were the one betting then you have to show