#Sink Holes
In addition to the relatively innocous explanations others have offered, there
is also a possibility that some amount of that gentle hillside is getting ready
to slide off in the direction dictated by gravity. I've forgotten the
technical term — something involving "slumping" I think. I think it's
uncommon, but it's not unheard of. Back in the 1950s that happened to a whole
housing development in Southern California. One may not want to buy the
property if it getting ready to move elsewhere.