No idea. This certainly never happened to me, and I have not heard that others experienced the problem.

Did you buy/download you copy from MakeMusic/Finale directly?

Perhaps a reinstall would help.

You can specify which app opens all documents of a particular type. How/why this happened is unimportant. The fix is easy.

Back in the old days, many apps used the .mus suffix for their files and this was quite common.