I did a new Fullrender with Dynmap on one of my worlds because I updated to Dynmap 2.6Dev. Then I found a couple of single chunks outside of my Worldborder. This happend with older versions of Dynmap before. I´m sure there was no player near this area.

Any idea what can cause this?

Sorry, no idea. I've seen Minecraft do that particularly when generating a new world, and less frequently later on, but I don't know what specifically causes it.

Just a theorie:
This did not happen when the server was on 1.8.8. I updated in april on 1.11.2.
Dynmap loads/unloads chunks while rendering, maybe this is caused by new spawned skeleton-horses?
All these single chunks are near the worldborder.

i have the same issue https://imgur.com/a/KDYzgvm

Spigot-a1f2566-97b0057 (MC: 1.13.2) (Implementing API version 1.13.2-R0.1-SNAPSHOT)
Dynmap version: core=3.0-SNAPSHOT-145, plugin=3.0-SNAPSHOT-145
WorldBorder v1.9.0

i saw this also without a worldborder (example https://imgur.com/a/88ZTajy)