Mine do that too sometimes really bad. I have had plants as much as 2 inches or more below the soil. I guess it is our freezing/thawing and rain and such. One thing I noticed last year is that some of my plants had a mole or vole run underneath them (couldn't tell from the top) and then later heavy rains caused it to cave in and the plant fell through and started to rot or heaves up and dries out (mostly iris do that) such a pain, I hate moles.
