How many years old is the home? A lot of times when a home gets about 10 years old, you'll see this if the builders buried the lumber they cut down. As the lumber decomposes it creates a sinkhole. Actually happened to my own yard and took 14 yards of dirt to fill it in. That was 5 years ago. So far so good.
My guess is a old stump that was there when the house was built, the part underground finally rotted enough to cause a sinkhole. You will probably have to fill it in a few times, just fill and pack a little bit and repeat as needed until it fully settles.
After I read all of reply's, I had to think you were in MD.
I would almost bet it's a sinkhole. Around Frederick county,
there is alot of limestone. and w/ all the droughts, then all the precip we've had the last 1-1/2 years, the ground is shifting alot.