Just because the weather has been so mild.
Have you overwintered it like this before? I'd be surprised if a plastic bag would do the trick if we get much very cold weather. At least put a good thick layer of dry mulch round it. Bubble plastic or fleece might give you more protection?

I agree, but plastic can lead to condensation as the plant continues to respire. the wet in the bag may lead to rotting. fleece would be better really.

Another thought - someone recommended to put a large earthenware flowerpot over a salvia - how about trying that - it would avoid condensation problems.