Astarion does not have permission to take baths. He is punished if he does. However, not taking baths means he gets grimier and grimier, and he gets punished if he's too grimy. Furthermore, being too grimy means that he is much worse at attracting victims for Cazador, which also earns him punishment. Pointing out the inherent injustice of this system also gets him punished.
Sometimes Cazador punishes him just because he feels like it. Astarion is of the opinion that this isn't really "being punished" so much as Cazador just being a sadistic motherfucker who likes to see people under his power suffer. Astarion has not voiced this opinion because he has more than two brain cells.
So he's taking a bath while Cazador is away somewhere, because even though he is going to be punished for that he has reached the point where going any longer without would incur more punishment. And it's while he's luxuriating in Cazador's fourth guest bath (the one reserved for guests he does not want to host at all), perfuming himself and just relaxing in the water, that the nautiloid shows up in the skies of Baldur's Gate. And because Astarion is having such a grand time in the bath he entirely fails to notice the Nautiloid until it's ripped the ceiling off Cazador's manor. He barely has the time to ponder life's injustices before one of its many tentacles brushes against his skin, causing him to be instantly teleported into a disgusting fleshy pod inside the illithid ship.
He supposes he will ponder life's injustices from within his pod.