I've done a lot of thinking about this lately. The triggering event was a health crisis for my mom that ultimately has been resolved, but it was very serious at the time. My father has a degenerative disease, and when she was in the hospital and unable to help him or cover for him, all of us were able to really see how poorly he was really doing. Plus he's continued to decline since then.
All this triggered fear in me that I will become a burden to my own family. We've got a busy family life, and I already feel stretched thin, so a serious illness feels to me as if it would be disastrous.
I also feel a lot of guilt, and as if I don't have a lot of control over my environment and circumstances.
So I think it's a triumvirate of fear, guilt, and a lack of control, all triggered by witnessing serious and progressive illness in someone I love.
Oh, and my family has a history of anxiety and what is probably mild OCD, including some hypochondria in the family tree. I think I've probably always had a bit of anxiety, but I was able to manage it until this past year.
I read in another thread about the guilt of making someone else's diagnosis about oneself -- ugh, that is me. I hate it and tell myself to get over myself.