My sister-in-law was having a weird condition where her car would intermittently not start, so she brought it so me and I checked it out for her. Turned out to be an easy fix. The big wire on the starter was barely on! The nut was just hardly on the end of the stud. I tightened it down and it was all better. Simple.

 

What I'm struggling to figure out  is HOW it came loose. Any ideas?

Probably wasn't torqued right.

Agreed. How old is the starter?

Wouldn't be surprised to learn that the starter had been replaced since car was new. Tech prolly finger tightened the nut, but then later forgot to tightened the nut properly with a wrench. Over time, the nut loosened up, and backed off, and that is the condition you found it in.

Sounds pretty reasonable, and I had assumed that this was the case. Glad to hear that no one thinks anything strange is going on!