Rumsfeld came in with an complicated mandate and agenda: reform the DoD from the inside out WHILE fighting the GWOT. A lesser man would have never attempted it, or resigned long ago. Rumsfeld is neither a lesser man nor a quitter.
Meanwhile the DoD was chockablock with general officers who had ascended within the old structure and therefore had a deep personal investment in it. The final years of their ascension came on Clinton's watch. They had reached a state of ease, a certain understanding with Clinton.
Rumsfeld came into the generals' "house" and promptly started throwing large pieces of comfortable furniture out the window and changing not just the structure on top but the foundation it rested on.
Some generals, such as Franks and Myers, saluted smartly and started working closely with Rumsfeld to implement the new plan. Others felt left out or even rejected. They helped fight the initial battles in the GWOT, but their hearts weren't entirely in it. They weren't part of the favored crowd any more, and they started to gripe and complain to one another.
Once in private life, they saw that complaining about Rumsfeld and Bush was a goldmine. Now they're mining it, shilling books, casting about for political sponsorship, and being invited to the best parties in Washington DC, New York City, and Hollywood.