Thanks Jay. You and Howard were right. It was the finish (System3 WR-LPU)

Sanded, reapplied 4 thin coats, looks like new again.

That S3 WR-LPU is harder than Chinese arithmetic when it dries (as opposed to spar varnish which maintains a little "flex" for wood movement)
I think maybe it was somehow stressed enough in some places to cause all those tiny cracks.

Thanks guys.