This mouse actually has Silly Little Guy (SLG) syndrome. It's not fatal, but it is lifelong in most mice.
I've had a few pet mice, and some of them just like to have fun doing silly shit like this. I've seen them go really fast in a wheel, then hang on so they whip around on inertia, that guy also had SLG, but lived a full life.
Oh yeah, definitely this. If it doesn't break down in water, it won't break down in the pipes.