Anxious Shower Dog

My dog was an anxious wreck when we rescued her. In the past 4 years she has mellowed out but still retains some strange behaviors.

When we are home she prefers to chill alone most of the time. She’s very secure on her own now. However, if I take a shower, she slams on the door and claws to get in. If I let her in, she cries to get out almost immediately. Every time!

Is this just an unexplainable idiosyncrasy? If you have any guesses on what might be making her nervous I’d love to hear it. I want to think of ways to make her more comfortable as this seems stressful for her.

Thanks, all!