That and, let's face it, up until that point Achilles never really had a *reason* to be truly angered. He'd had a truly charmed life, by the standards and ideals of the era and writer. And then someone comes along and butchers his best friend/lover/whatever they were exactly. Yeah... he got a little pissed... and yeah, he paid for it, because of his arrogance and pride.
In the Thrawn series- they had *none* of those allusions, buildups and potential flaws. There were no hints. No possible justifications or leadups. Now, while I agree that some things can come out of nowhere (the Grand Canyon- a mile deep cliff in the middle of a rather flat field- who'dathunkit)... but, seriously, it was just not good writing. The guy had already proven himself superior to far better plans and surprises than what eventually brought him down. This wasn't even "War of the Worlds" random defeat... this wasn't even "rock falls, everyone dies"... this was something on par with "batman gets ass kicked by ordinary unarmed thug".