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Latchup is often triggered by current injected accidentally from a well. If the current is high enough to give a 0.5-0.6v drop between the source and the closest pickup, it can begin turning on parasitic bjt.
Thinner epi will have a lower resistance path to substrate and therefore can tolerate more current before the resistive drop reaches 0.6v turn-on of the "base" of the parasitic bjt.