How long until she had him soaking in the tub before she decided to leave the room? Did she put hum in the tub to soak and immediately left the room and came back in later? If so, then my guess would be that he picked up the vibrations and perhaps that spooked him.
If she had placed him in the tub and continued to stay in the room for a length of time while he settled down then left later I would say it's "me so haaaawny" since if he had been settled down in the tub soaking for some time and then she gets up and leaves the room it would have set him off then.
But I'm no snake (depending on who you talk to) so it's all conjecture.
Now, if she had set the snake in the tub to soak, immediately left the room then snuck back in wearing those big, fuzzy, lady slippers with the non-slip soles made in China that look like a cat regurgitated them several times over the years then...okay. I'll admit it. I'm beginning to digress here. Forget you even saw this paragraph.