A childhood belief about toys

When swinging on the swing set in the playground, if there was another kid swinging in the adjacent swing and the timing of their swinging became synchronized with mine, it was understood that either of us were obligated to shout, "Get out of my bathtub!" When I looked up this quaint custom on the internet, it seemed to be an actual thing that kids did in the past. Apparently it's such an old custom that the original phrase used to be "Get off my party line!", which of course referred to multiple people having a conversation on the same telephone line. I suspect that this etiquette arose because a swing set can be destabilized, and may tip over when two or more people are swinging at the same frequency and height, and so shouting the phrase was a request for the other person to adjust their own swinging frequency.