My bet is they had some inkling of it. Chavez must have had these Brownshirts around for a while and the military should have known they were potential trouble. If they didn't think of it, they were remiss. If we knew Chavez was trouble by observing him from this country, they must have realized it even moreso there.
I think they just tried to do the best with the cards that were dealt and weren't prepared.
You're probably right. They weren't prepared. They should have been smart enought to know you don't ask a Marxist to "resign". It's never successful, so they should have put together a better plan before they moved. Once they moved, they should have gone all the way - not take half-measures.