This question really sums it all up, doesn't it?
"what is it about George Bush that seems to reduce once sober and experienced liberal pros to infantile ranting?"
Really. These elitist snobs are acting like hurt, cornered animals. They are hysterical & DON'T act like a party that is "comfortably ahead in the polls". They know more than they are letting on, they know they are losing. Now is the time to go in for the kill.
Actually, I think the answer is fairly simple: they regard GWB (a Yale and Harvard grad) as a traitor to their class. Not only didn't he become a liberal, he didn't become a tame Rockefeller Republican and stay in the northeast with an affected patrician accent. No, he went off, adopted Texas as his home, got a Texas accent, and became something approximating a conservative Republican, complete with appeals to God in his speeches, made worse because in unprepared remarks, he makes it clear they are sincere.
W rejected their culture, so even his unconservative embrace of entitlement programs and immigration amnesty doesn't win him any good will from them.