I'm going to cast my vote for mole. Can you pull the mulch back and investigate with your hand? You may have a tunnel to fill...I've had several of my own this year. Problem is, if you can't dispatch the mole (s) and this is a regular run, they will open it up again. (The mole tunnels can also drain any irrigation water away too quickly for it to be of much benefit.)

They do eat grubs of course, but the mole killed here this year had a tummy packed full of nightcrawlers...all that compost I've been adding for years? Not that an autopsy was done, but if I told you how we knew what the diet had been I'd have to describe DH's aim with a foresters hoe. :)

I have a different theory as I experienced the very similar situation few years ago.
Before I knew any better, I planted few azaleas without tearing apart their pot bound root balls. Though I planted them 1-2 inches above the soil level and put mound of soil and mulch on top of it, next year they were look like they were had been pushed up similar to heucheras heaving and mulch was washed out of the root zone. Plants didn't grow an inch for the whole season.
I dug them out and of course root ball was in a same exactly 'pot shape' as it was a year ago. Also, it was much drier than surrounding soil.
After all was done properly they took off and growing happily ever since.