Volume gets maximized

Nearly every day the ring tone of my phone is configured to be at maximum. Only the ring tone volume, the other volumes stay unchanged. I'm searching for the reason.

My first suspect was Motorola Assist. I'm using the night silence function. So I froze the app and use the standard android function for this under settings, but it didn't help.

I also wonder why it happens not every day, but about every second. Usually at night I guess.

Any ideas what could be the cause?