Might be related to the Masque's antics during the Fall of Biel'tan. But also eldar whom snapped totally to delve even deeper into depravity and excess than even the drukhari, making deals with slaanesh to save their own skin or just plain ole lust for power. Especially if their ambitions repeatedly has been checked by Asdrubael Vect and leading to a too deep resentment and feeling of entitlement. There was that Soul drinkers novel with a deldar archon doing a slaaneshi ritual. At the time it seemed far fetched, but now? Clearly there is eldar whom have gone off the deep end.