Hi
Whilst I can't say why that is happening, the following steps may help to prevent the issue from recurring:
If you have not yet done so, update your iPhone to the latest software:
Restart both your iPhone and your Apple Watch, turning both off together, then restarting your iPhone first:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- Restart your Apple Watch - Apple Support
- If the issue recurs, close open apps on your iPhone and then restart both devices again.
Unpair and re-pair your Apple Watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
- If the issue persists, consider instead unpairing and setting it up as a new watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support
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 with a view to having your watch checked and, if recommended, serviced: