Why would a three phase motor suddenly change it's rotating direction?
You blew a fuse somewhere ahead of this pump and are now giving it single phase power. A 3 phase motor will not start with single phase power, but it will run if started some other way. Your pump likely does not have a check valve to prevent water from flowing backward, so when it is off, there is a slight back-flow spinning it backward, even if only slightly and when you energize it with the single phase power, it is already turning the wrong direction, so it continues to do so.

Find and replace the blown fuse.