Tav uses the tadpole link to connect to Astarion's memories of being abused by Cazador. Either to use as masturbatory material or just out of curiosity. This means not only does Astarion have to relive them, but he's also aware of Tav witnessing them and maybe getting of to them.

I would prefer a non-Durge Tav for extra creepiness. Maybe a Lolth sworn drow who has internalised that men can't have agency (but this isn't necessary at all).