Friends of mine coined the term almost 40 years ago, one had a 1970 Nova that did the same as my 66 Belvedere, just a normal random thing.
I haven't had this happen in over 30 years, but remember it well.
Chances are the nylon clip lock post through the lower window support simply parted, going the way of all plastics.
I replaced mine with a metal bolt, wrapped in teflon tape where the stress rides, this on a 72 Dart, no problems in 8 years with this repair.
The window support post is about 7/16 nylon, you might be able to center drill it and put in a smaller metal bolt to restore it.
Best wishes and happy motoring.