It is possible to be pulled suddenly. Happened to an ex colleague before, and she is not a low performer. Each year, the principal will need to declare personnel in excess based on the teacher quota for each subject. This is based on HR rules. It used to have an additional layer of job grade but not sure now. There is some complexity involved and principals will usually declare in such a way that reduces the chance of their teachers being pulled out of the system. If you find that you belonged to a subject where there are too many teachers that are hard to deploy then there is high chance you are in that list. Most Heads would have known who in their department has been declared as excess too.