Can he see out of the tub? Is the plastic opaque, foggy or clear?
I ask because it may be that your presence may suddenly seem to loom over him & in his position, that's pretty scary. But even if he can
see out, snakes don't see that well, nor do they identify things by sight. They see motion & that registers either as "prey" or "predator"...
& since you're pretty big compared to him, he assumes you're a hungry predator.