Apple Watch pinged a phone that is not mine. Why, how do I replicate, and how do I avoid?
The watch only pings one phone in my experience (but many watches can pair and ping to one phone)- the phone it paired to from the Watch app. I would first start by looking at each phone in question - check which watches are paired to which phone.

To confirm, I assume are using this tap to summon the phone.

On the fixing, I always like to unpair the watch on the phone I have as primary. Then back that phone up to iCloud and to disk using Finder / Music / iTunes. That way my health and activity data are all in one backup. Then I re-pair the watch with the phone I prefer.