Re: The Chicxulub impact
At a guess? Miscalculation. Too much damage to the ecosphere; the resultant environment was too harsh for the species they wished to introduce.
So, they went away, intending to come back once things had stabilized, but their civilization didn't last that long.
Of course, the first xenoarchaeologist to correctly translate their starmaps is going to have a real uphill battle getting anyone to believe it...