That is the puzzle isn’t it? Most rings As everyone knows are hard to insert as the 360 overlays want to stay together.
I guess I can see the fibres in a rope getting caught and getting it started, or the ring being brittle and breaking off etc. but that doesn’t seem likely in your case. Hmmmm
OK this thread is starting to go around in circles, but I'll say it again. RINGS AND PINS DON'T JUST FALL OFF.
They break, or they are pulled off, normally by lines sliding by. Rings are much more susceptible to this, and that why I only use pins a the deck level, or where a line can pass. There is an open debate about the 15 degree opening, or the wrap. I chose to wrap (and never reuse) pins at the deck level. I know in theory this slightly work hardens the metal, but have never seen any evidence that this has cause a pin to break. In any case, inspect ever time you sail.
On a backstay, free of lines to snag it, a properly installed ring will never ever 'shake off'.
I head tons of stories like this; its a cover for a lack of inspection every time.