Because they can't cope with the guilt and shame of what they are doing, as well as wanting to be free of all responsibility. They feel like they are being choked, etc. They honestly can't give you a valid reason for leaving, they just know they have to do this right now, no matter the consequences.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
