Sessions isn’t a stupid man so, we can rule out mental retardation.
He could be a member of the swamp club
or,
Most likely the swamp showed him his NSA file and said something like,” ooh Jeff, wouldn’t`t want the family and the public to find out about all those credit card charges to sexyladyboysall4u . com now would you? Why not just recuse yourself even though there is no reason to and go play in the little boy sandbox with the DoJ toys and leave the big stuff to us and Rosenstein.”
.
Sessions is neck-deep in Uranium one.
That one cost several Republicans their gonads.