Q: Apple watch sport restarts unexpectedly when cover to mute call?

Have you tried a force restart?

Press and hold both the side button and Digital Crown for at least 10 seconds, until you see the Apple logo:

Restart your Apple Watch - Apple Support

Even when I put my hand on the clock, to cover the display of the Apple Watch, this restarts unexpectedly.

Apple Watch Sport and iPhone 5 - 32GB with iOS 8.4

I'm italian

Hi Kabal & Ashhh

• Riavvio di Apple Watch
• Riavvio forzato di Apple Watch

Riavvio di Apple Watch - Supporto Apple

It seems that others are also experiencing the same problem. One person noted that the feature worked fine in watchOS 1.0 and has only caused restarts since he updated to watchOS 1.0.1 (current version).

Re: crash when muting call on Apple Watch

It may be worth contacting Apple Support:

https://www.apple.com/support/watch/contact/

Or at least letting them know that you are also experiencing the same problem: 

https://www.apple.com/feedback/watch.html

watchOS 2.0 is due for release this Fall / Autumn.

I resolved with the update of the Apple Watchos 2.1