So it was revealed that Mark's wife is Ms. Casey inside, but outside, he believes she was killed in a car accident. My theory is that his wife was already part of the Severed program at the time of the accident. Her brain was split into these two separate halves of work and personal. The car accident caused brain damage to the personal side causing her to be in a vegetative/brain dead state. However, the work side of her brain was undamaged, so as soon as she arrives on the floor at Lumon, she sits up and gets back to work. Mark gave the go ahead to pull the plug on the outside and perhaps thought she was cremated. He's depressed and joins the Severed program as his way of dealing with his grief. Where he now works with his wife and doesn't realize it.