How big is the hole? I have had gopher holes open up that seemed to soak up a ton of water. The gophers in my backyard had free reign until I moved in and created an extensive set of tunnels.

Not sure if the roots are being eaten, but all of these plants have been doing worse than last year, although it could be unrelated. I'm sure having part of the rootball exposed doesn't help.

I don't think it's gophers, but I would not be surprised if it's moles or some kind of insect like a grub . Big nasty looking grubbs are all over the place here. Do moles eat grubs, and would be attracted to all of the beetle grubs in the area?

Earthworms or some other natural process wouldn't cause this?

According to Wikipedia moles do eat small invertebreas and worms. I haven't ever seen earthworms make holes as they are mainly eating dead organic matter.

I would look hard for gophers (What we have here), moles, or voles. I have seen too many of my trees with the same problem here.