Mine do it when it's warm with plenty of water during the early, faster growth stage in the life of the plant. My brother-in-law who grows some in the Houston area in the ground where it's warm and wet naturally says his clumps do it as well. I can't say I like it, but I don't worry about it anymore. Today and yesterday we've had rain and cool weather, so they aren't doing it much. If the sun comes out and it warms up this afternoon, I'm sure I'll be seeing the new center growth fall over and maybe a wilted look to some entire clumps. Mine just do that.
