There needs to be plenty of surface area in your water softener drain pipe to ensure that water can sufficiently flow out of the system when it regenerates.
If your drain pipe becomes blocked, excess hardness minerals, iron and dirt will accumulate in the softener resin. Your resin may become so damaged that you end up having to replace it fully, which is an expensive job.
Severely restricted drain pipes can even cause water softeners to blow off during regeneration, causing a flood.
The solution is simple: make sure your drain line is always free of blockages. If you suspect a blockage, investigate it and clear out the pipe immediately.