Wow, I do that as well, so you're not alone. I remember there was a time when I was afraid t o shower because I would always have an attack in there for some reason. It's tough!
The shower puts you in a vulnerable place/position. One where you feel you couldn't escape a panic attack and may feel "trapped". Plus, you're in a very small space which in itself can trigger a panic attack.
I too panic in the shower. I think it is because its a small place.