Either a seal or piston or motor problem.

Unplug the softener and put the softener in bypass or that water running to drain will add up on your water bill, You'll be using untreated water until you resolve this problem.

Most likely the softener needs to be rebuilt and if old the resin replaced. May cost more to repair than replace with a new industry standard softener that will be more efficient than what you have now.

What model and how old is the softener?

Has anyone been doing routine maintenance on the softener?

City water or well water?.