Sometimes some of your worms migrate to the sump which is mostly more distressing for you than them to be honest. They do explore their surroundings and they are usually fine in there as long as the liquid is draining reasonably regularly. The reason they go down to the sump on occasion isn’t really known – in winter it’s possible they are heading down there to keep warm as in their natural environment they do burrow deeper to below where the ground is frozen. Either way generally this isn’t a concern. If it is a real problem add some crumpled up newspaper and leave the tap open.