Why does it keep asking to Undo?

Undo functionality is provided across Watches. If you make a mistake (say, a false start or tapping the wrong person’s lap button), just shake your device vigorously. However, that means that if you shake the phone inadvertently (say, jogging across a field to the next event), the phone thinks you want to Undo a command. If this bothers you, you can disable this functionality across your phone in Settings > Accessibility > Touch > Shake to Undo.

Unfortunately, that would lose a powerful ability, but you can then enable BackTap in that same screen to make tapping the back of the phone twice or three times to mean Undo instead.