In my case, this seems to be triggered by a player who keeps trying to log in. It's possible that as he logs in, the chunk doesn't load and he drifts farther and farther outside of the chunk.
See here for logs:
http://spigotmc.org/threads/36842
Also seems to be related to this issue:
https://hub.spigotmc.org/jira/browse/SPIGOT-58