Apple today alerted some developers about a temporary signing issue that prevents their apps from being installed on the Apple Watch.
The full text of Apple's email, obtained from a MacRumors reader:
We've discovered that there was a temporary issue with the App Store signature of your binaries that affects the ability of your app to install on Apple Watch. As a result, we're pushing an update with the proper signing to customers' devices.
It's unclear what went wrong, but Apple is pushing server-side updates with the proper signing to the devices of affected customers.