A blocked or frozen drain line would cause not water flow through the drain line. That would prevent any suction of brine water from the salt tank and when the softener went into regeneration, no water would be sucked out of the salt tank and then more water would be added by the refill position of the regeneration. It will continue to add water until it overflows or a float stops it.
With no salt water being used, the resin will not be regenerated. So when you get water flowing out the drain line let it use the water is in the salt tank instead of emptying any out of it.