Maybe it's just normal dissociation,no different than when someone drives somewhere but doesn't really remember driving there?

Showering is such a mundane,repetitive thing that doesn't really require any focus,so maybe you're just so busy thinking about other things that you don't even pay attention to what you are doing..just doing it automatically,just like so many other things we all(the entire population)do on autopilot.

Or,maybe it's possible there's past trauma related to showering.I used to really struggle with showers/baths,panic attacks,flashbacks,etc.,but once I worked through the trauma the struggles went away.