Hi
It is not normal for your Apple Watch to turn off unexpectedly.
Your Apple Watch may require service. Whilst I cannot say why the issue is occurring, before contacting Apple, it may help to follow these steps:
If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:
Restart both your iPhone and your Apple Watch, turning both off together, then restarting your iPhone first:
Check for potentially problematic apps:
- Remove third-party apps from the Dock and/or from your Apple Watch and then monitor battery performance after adding them back, one at a time:
- Remove watch face complications and monitor battery performance as you add them back:
Unpair and re-pair your Apple Watch:
If the issue then still persists, I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider with a view to having your Apple Watch checked and, if recommended, serviced:
Apple Watch Service Answer Center - Apple Support