"Never heard of lean mixture causing preignition though."

On the contrary, lean mixtures can cause preignition. The combustion chamber runs hotter on a lean mixture. Heat leads to detonation (pre-ignition). You will notice all the performance people recommend fuel enrichment to prevent detonation. Fuel mixture gauges are sold for this, and many people with highly boosted cars run additional injectors (triggered by boost pressure) to prevent lean mixture problems.