"These men were of an age to have studied Rawle on secession, and were certainly capable of making up their own minds"

Exactly my thoughts.

What leads such men make such drastic decisions?

These men were not uneducated rubes. They had devoted decades of their lives to the Service of the United States. They had placed their lives in danger to fight the wars of this country and in doing the thankless duties of living on the frontier in out of the way outpost.

No these men were the extraordinary. They were the best that this nation could produce at that time. And were as good for their time as any professional soldier is today. They were not mindnumbed robots that left the army after so many years to become a part of something which could be thought to be treason.

Remember Lee commanded the force that captured a Traitor, who was later hung for his crime, John Brown. The radical republicans made that Traitor a national hero.

No their decision were not made lightly. They were made with due consideration and much soul searching and made the decision that they considered to be right and just. That decision cost many of them their lives. The fact that they lost their military struggle take nothing away from the reason that they made those decision in 1861.