Join another Zwifter/super late group join triggered mid-ride

I couldn’t make up my mind what ride to do this morning; using the companion app, I’d joined a group ride.
When starting the game (MacOS Monterey, needed a system restart to get bluetooth working, so missed the start of the ride) I chose the route France/Tire Bouchon, but then noticed the friends name in the Join another Zwifter list and clicked on it. After clicking to join their name, I remembered they were on a group ride, so chose Exit/Delete, and went back to the ride choice modal menu. My memory is hazy - I might have cancelled joining the group ride when I realised I’d miss the start due to the Mac reboot.

This time, I chose France/Tire Bouchon and started to ride. The distance to end of route progress bar was shown counting down, and after just over an hour (at least 1:05), I was just over half-way around (after the big hill!), expecting to take another hour to complete the route for the badge.
I saw a big ride approaching, heading in the opposite direction wearing the same burgundy strip, and a black message briefly flashed up on screen “Join R…” - and “Pedal assist ends in x seconds”, and my avatar was relocated on the map.
The progress bar for Tire Bouchon had disappeared; I’ll have to start that route again another day!

So, my question is - why did “Join another Zwifter” trigger as they rode past me on the map; or “late join group ride” after over an hour (note: i was not seeing Join ride button bottom left during my ride)?