Geese and Soil

Over the past few months our geese have started to eat the soil around the pear trees and now there are big holes at the bases of the trees.  I have been filling in these holes but I'm wondering why it's happening.  They have grit available and don't eat the soil around the other fruit trees (well, they started on the Mirabelle but quickly came to prefer pear).  Do pear trees exude some kind of thing into the soil that they might be deficient in?

I wonder if there are grubs feeding on the roots, that the geese have learned to dig for?

yeah, my geese do the same thing, doesn't seem to be a whole lot of rhyme or reason to it. I thought at first it was a nesting instinct, cause they were also trying to breed and the way they dig, it's like they are in a carnal frenzy, very similar to their mating instincts. However, i thought this when they were initially digging small pits into a wood chip pile.
Later on however, the little buggers were digging pits in filthy mud wholes at the entrance to a barn where many animals routinely evacuate their stool and urine.
They often get their faces absolutely covered in muck, looks strangely tribal at times haha, geese with war paint!

yeah, perhaps there are creatures under the wood chips and soil they are eating, but i have never noticed my geese eating any kind of critters at all, so i can't attest to that.