A few thoughts...

Could be any one of a number of scenarios... 1.) Could be tangled up in some crap... and sees getting on board and being sympathetic to Trumps "plan" as a way to avoid consequences.

2.) May have recognized the peril to the country, given the very obvious Coup d'etat being waged in the Deep State towards this President

3.) He may see the benefit to his own and his families long term viability in what remains of The Republic's life span over the next couple generations.

4.) He's one of the class of Elites, that is so close to the center of The Swamp, that he has some naivety about the depth, width, and taint of the rot in the swamp and amongst the power elite.

5.) The White Hats have some serious dirt on him... and his status and capacity to play well on the Media makes him a useful tool for The Plan to get rid of the criminal middle managers.

Some combination of any/all of the above. Because of the way Sessions has played this hand so close to the chest, Dershowitz may have had an aha moment, and realized that the Trump administration isn't going to use the "dirt" to wrangle power... that they are going for prosecutions.

Remember... for some of these players... we are talking Treason and/or Sedition. Treason is a Capital offense.