For my kids' freaking out, that is, beyond the simple fact that my Mom is here now, and things are just different when Gramma's here:
The last three times my mother has visited, I've had surgery.
It wouldn't surprise me a bit if the kids subconsciously associate my Mom being here with me needing to go into the hospital again.
I'm hoping I won't have to, of course, and I don't have anything scheduled right now. Here's hoping nothing comes up to change that.