I like to think that there was an off-screen, nasty run in with klingons right before Praxis, making the older crew a little miffed.
IIRC, the novelization for STVI has something exactly along this line. Some sort of klingon attack/massacre on a Federation colony world. Been a long while since I read it, but I seem to remember a scence with Kirk visiting a horribly injured Carol Marcus in a hospital.
An event like this right before what we see in STVI might explain some of the attitudes of the Enteprise crew, I think.