Hello, aurelien48 and thank you for using Apple Support Communities.
With no clear reason as to why it happens, you'll want to start narrow things down to determine what causes the Mac to restart. This article has some good steps to take to help with that: If your Mac restarted because of a problem - Apple Support
"Check your software
If the issue is caused by software on your Mac, one of these steps might help:
Check your hardware
Learn how to check your connected devices and other hardware.
Check peripheral devices first
If you don't have any devices attached to your Mac, skip to the next section.
- Shut down your Mac.
- Disconnect all peripheral devices, such as hard drives or printers. If you have a desktop Mac, make sure that only a display, keyboard, and mouse or trackpad are connected.
- Turn on your Mac.
- Use your Mac for the amount of time that it would usually take for an unexpected restart to occur.
- If an unexpected restart occurs, follow the steps to check the internal RAM and third-party hardware.
- If an unexpected restart doesn't occur, turn off the Mac and connect one peripheral device at a time until an unexpected restart occurs.
Let us know how it goes and we'll go from there.
Cheers!