CroCop says it wasn't conditioning; he was tense, he froze, and had an adrenaline dump. in other words, nerves.
that being said, it was his 13th fight in under 2 years. things were different then.
but ya it's weird watching these "amazing" fights later. it wasn't that this particular fight was **exciting**, per se; it was the context of the bout. and that context gets completely lost after live. not unlike two guys not in the top 20 can have an "exciting" bout, but it's totally meaningless in the grand scheme of things. does that make it more or less fascinating than a top 5 v top 5 stylistic matchup that had less action?
i remember watching Wandy v Rampage II and thinking it was the best fight ever. it was the one that left Rampage in the ropes dripping blood like a waterfall. years later i realized it was half on the ground with Rampage on top doing very little. so about half the fight was the best half fight ever
but overall i'd completely forgotten the slower parts had happened.