Culligans are famous for having rubber gaskets and selenoid valves getting stuck. The rubber seats are rotted by hard water and there is no way for the unit to seal itself off from flowing water to the drain. The whole valve needs to be rebuilt

Sometimes the heads go bad on those and they get stuck in a regeneration cycle that just flushes the tank out. put it on bypass or call your softner maker and alot of times they will help you diagnose it and send you replacement parts. also some of the softner heads use small electric motors that get siezed as well