well for mine, they are almost always plants that have been planted within the past year, new ones. I can only think that loosening the soil when planting, and then planting, later the soil settles and the plant sinks. like after a big rain. I also think lots of compost does it. The compost breaks down and the plant sinks. Next, I have had plants fall down into the soil and upon further inspection found that a mole had a tunnel directly under it. When it rained, got soft the plant fell in. Could not tell why from above until I dug into it and realized it was hollow underneath.
