Sinking Feeling

A few months ago, I noticed that some of the flagstone on the left side of the house seemed to be sinking a little. Then, more recently, while setting up the new trellis, I really noticed that a strip of soil was DEFINITELY sinking.
After removing the stones and compacting the soil, I ended up with a small trench (up to 12" deep) along the edge of the deck (NO soil was removed, only compacted).
What the heck?
I just hope this is just a simple case of soil settling over the years and NOT a deeper problem.