I have good news for you. I just looked it up, because I thought the same as you.

It's just playing and/or showboating.

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.

That's a hamster, not a mouse, and they're basically retarded. I know we're not supposed to use that word anymore, but I really can't think of a better adjective.