I'm wondering how many of the TPKs happened in groups where the DM didn't read the encounter properly.
spoiler: the kobolds inside the cave possibly are surprised, and the elites, including Irontooth, don't show up until 3 rounds after the PCs entered the cave. Unless, of course, a kobold fled from the previous encounter and warned the kobolds inside, in which case they are ready for the PCs and the 3 rounds clock start from the moment the kobold fled inside to warn the others, rather than from the moment when the PCs entered.