Didn't the other spawn tell Astarion that Cazador has his eyes on him and knew all along where he was, etc. Then, at the mansion there's a diary or something where it's clear that the spawns at some point told Cazador Astarion is coming for him. Which would mean Cazador knows Astarion is coming and therefor he has prepared everything for that moment, so that when Astarion does arrive, he can just send him to his spot and proceed with the ritual, which he does because he can still control Astarion.
I'm not sure how this refers to my question. Cazador magically blocked the way to the dungeon, to make sure no one will come through it - and he did this before Astarion arrived so he intentionally blocked Astarion's way. Also, the ritual already started at this point, as Godey and Cazador's servant say in the game.
