My RAM was bad ("three beeps" bad) until shuffled
I couldn't possibly venture a guess as to "why" except to chalk it up to Murphy...

However Mac O/S is very picky when it comes to RAM. It is my guess that Apple updated some of the code in the O/S that checks for issues with RAM and it noticed a problem.

That it now works with the RAM effectively swapped is even more puzzling. It is possible that there is an issue with something in the RAM slot itself, either physical or "electro-mechanical."

If the RAM is within warranty I would contact the folks you bought it from and swap it out on the chance that there is a strange issue with the RAM itself.

Sorry, no real definitive answer. It could be just a random occurrence and if you swap the RAM again it could work fine. Or the Earth could veer out of it's orbit and go crashing into the sun... Sometimes troubleshooting hardware and software issues leads you down a road that makes no sense, but if your "fix" works then leave it be and hope for the best.

Any any time I hope for the best, I make sure I have a recent backup...