Some people do it because they're overwhelmed, can't cope with their emotions, never learnt/were never modelled good conflict resolution and coping strategies as a child.
None of that matters. Once you communicate once that it's not acceptable, they should change, or you should leave.
Withdrawing to process their emotions, for a short time (usually hours, max 1-2 days), and communicating clearly that that's what they're doing, and that they'll be back and will pick up the conversation then, is mature and fine.
Silent treatment, whatever the cause, it's not acceptable in adults.
There's research that shows it causes the same changes as physical pain in the brain. It's abusive, and you need to leave it he does it more than extremely occasionally, preferably only once.