I am having this issue also with segments accidentally going off (and sometimes the workout stopping) when I don't want them to. I think it has to do with wearing long sleeve shirts and/or jackets and getting wet.
Two things that seem to help are tightening my watch band prior to a run (avoid having loose space under the wrist) and locking the watch (see link for "End, pause, or lock your workout": https://support.apple.com/en-us/HT204523) once I have started the workout. Wish there was an easier way; it is irritating.
This is also in the "Adjust the workout settings on Apple Watch" section of the Apple Watch User Guide: https://support.apple.com/guide/watch/adjust-your-workout-settings-apde0be691be/watchos
Avoid accidental taps
If the exercise you’re doing or the gear you’re wearing causes accidental taps on your Apple Watch, lock the screen so your workout record isn’t interrupted.
- Lock the screen: Swipe right, then tap Lock.
- Dismiss a notification: Press the Digital Crown.
- Unlock the screen: Turn the Digital Crown.