The chickens run is developing a basement

[aart]

Need to see pics.
Could be rodents or erosion or both....or chickens are excellent excavators.

 

[Apocalypse]

All that dirt went somewhere... meaning there is another cavity somewhere, and all that dirt is getting washed into it.
Either animals, a leaking pipe or a natural feature has left a potential sinkhole in your yard from the sounds of it.
Pictures?

 

[Aunt Angus]

Pierce County gets a lil rain, I hear. Maybe that's the culprit? I've seen it happen where fine/loamy soil compacts and the looser stuff washes away. Happened in a garden we had at a school where I once worked. The topsoil formed a crust over a pocket. It was weird.

 

[msmeg26]

moles were my first thought.

 

[Cindy in PA]

We have vole tunnels all over our 2 acre yard this year. We normally get them when snow sits on the ground too long, but this year it has been constant since November. Might have something to do with the extreme wet year we had, but the whole yard is a treat to walk on, squishy & lumpy when wet & a tripping hazard when frozen. Never saw it like this in 35 years!