Doug , That's just tech speak for we don't know what the hell happened .
For being senior . I'm 61 & in good health so far . I count my blessings for this.
That don't mean I don't get the more aches & pains that I didn't used to.
I had a knee injury when I was a teen I got in a car wreck (the other guy's fault).
It occasionally comes back to haunt me. A bit of lower back problems too . I think I've hand pushed a few to many cars around.
On the other hand just me & my brother did a 4 inch by 8 foot by 6 foot concrete slab this week . Used one of those rental cement trailers & had to wheel barrow the cement about 50 feet into the back yard .