積もる is "to accumulate." Now you're doing the accumulating, and in order to do that you must go to pile things up. More and more and more. How do you know that this verb is when you're doing the accumulating? It's because you are piling up cows (む). Wow, a cow piler.
The reading comes from 積もる, so pile up your knowledge and remember it that way.
Now please grab a partner and begin loading the truck with turkey.