The drained boar appears to trigger the encounter, but...
I have never had more than one special event in a single night's rest, even if they were 'due'. And the prioritization for them is unclear. So what I think happens is that if the events are stacked up, they might trigger even if the cinematic and dialog don't.
Special events:
Astarion's reveal.
Raphael's offer.
The one you dream of (can't be more clear - spoiler - but you'll know it if you get it).
Other special events?
