After a couple week hiatus due to travel and other obligations, I finally got back to biking.
The first trip of 6 miles was with the G'kids in the neighborhood, then a second 28 mile trip.
Nearly halfway into the second trip on an uphill climb, I heard tick.. tick.. for about a minute, then the cassette locked up; I could not pedal forward nor backward. The cause? the locking ring holding the cassette had backed all the way out, then bound on the frame & derailleur drop, peeling a bit of aluminum in the process.
Fortunately, I carry tools and nitrile gloves. With a freshly oiled chain, I was happy to had gloves! The locking ring had bound at an angle in the threads, and resisted removal. I broke the multitool's needle-nose pliers in the process, and had to patch it back together to continue working on the ring. Finally it gave way, and I found the threads were still in great shape. I re-threaded the locking ring, then using the tips of the pliers, I was able to sufficiently tighten the ring. After assembling everything, all was quiet and stayed put. I have no idea why it backed out, after 4Kmiles+, this was a first for me.