I'm going to have to assume that some bug in the Minecraft server decided to generate chunks at super-distant random locations. Otherwise, someone lost their fracking mind on a portal run.
Based on these file names at the most extremes:
r.37.519.mca r.16.-505.mca r.-363.-34.mca r.500.12.mca
The map would currently span these ranges of block coordinates:
x from -185856 to 256511
z from -258560 to 266239
I plucked a few of these files manually to have a look, and here's an example:
Don't squint too hard! That's a mere 16x16 patch of grassland with no remarkable features. There are a hundred chunk files like this scattered across the map. I've wiped those out plus a couple of "legit" far-distant islands that were reached by portal, and I got down to this image (at 5% scale):
*whew*
I'll overlay my old Photoshop grid file with this and verify that everything checks out with you guys before trimming.