Welcome to Sober Recovery. You have found a really GREAT place, to vent, ask questions, and do some research.
You said "I cannot get a reason “why” out of him". He's not about to tell you yet that this is what alcoholics do.
Please get all the information you can on Alcoholism, and read all the "stickys" at the top of the home page of this, the Friends and Family Forum.
But yes, relapses are very common in those that do nothing but put the "plug in the jug" and attempt to make no further changes about themselves. The ISM in alcoholism is "I, SELF, ME."
Please keep posting and asking questions, you will find some really excellent Experience, Strength and Hope in this forum.
Might I ask if you have looked into Alanon yet?
Love and (((((to all))))),