Sometimes there is something when I was in the military, working in electronics, we used to call FM. the latter word being magic. Sometimes there just is not an explanation. Last year both a bay tree and a rosemary plant just up and died. Now I have to tell you, both were over a decade old and had been babied; not too much water, lived in the kitchen or bathroom in a sunny window during the winters. They were so darned healthy that my neighbors all knew that if they needed a bay leaf or some rosemary, they came to my house to get them. Then, all of a sudden, under circumstances they both thrived on for years (and yes, I'd repotted them several times), they just up and died on me. It amazed me because the rosemary had actually lived in the trailer with us! I now have a new rosemary and a new bay tree. But these plants were at least 6 years old, they were not root-bound (after I gave up the ghost I did a post-mortem. I think the bay had some kind of bugs, but even so, I'd washed the leaves in previous years with good results. So sometimes you just do not know.