jdub speaks truth - that was detonation, plain and simple. I'd wager with your lex afm + 680s, you pulled enough airflow signal out, that you were running too much timing -
Further, I'd wager it was *not* WOT runs that built up to this - but heavy boost under part throttle, where your timing is even further advanced than @ WOT.
Personally, I have no intentions of ever scaling the stock airflow signal more than 10%. If I exceed that, I need more air/fuel resolution, and a standalone is the only option for *reliable* power at that point.