Hi
It is certainly not normal for your Apple Watch to restart unexpectedly.
Whilst I can't say why that has been happening, the following updated set of steps may help to stop if from happening again (check performance after each before moving on to the next) - I appreciate that you may have already tried some of them, but it may help to try them again in conjunction with the other suggestions:
Check that, in respect of the instances that occur during wear, you are not accidentally force restarting your Apple Watch:
- If your watch is worn too close to your hand, the Digital Crown and side button may be pressed and held down by the back of your hand when flexing your wrist.
- It may help to wear your watch a little further up your arm and/or with the buttons on the other side:
- To change the configuration: on your iPhone, in the Watch app, go to: My Watch (tab) > General > Watch Orientation > choose the preferred wrist and side for the buttons.
If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:
Close certain apps and then restart (normally) both your iPhone and your Apple Watch:
Turn both devices off together, then restart your iPhone first:
Check for any known issues with any third-party apps that you may have noticed are specifically affected by consulting the developers' support resources and/or contacting the developers.
If the issue persists, next unpair and re-pair your Apple Watch again:
Test by changing notification settings and/or by removing watch face complications and third-party apps:
- Turn off notifications for third-party apps and test performance after re-enabling them one at a time:
- Remove watch face complications and monitor performance as you add them back:
- Remove third-party apps from the Dock and/or remove them completely from your Apple Watch and then monitor performance after adding them back, one at a time.
If the issue persists, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for further assistance: