With the plugin "WorldBorder" and the 1.9 Spigot there is a bug that teleports players to the worldborder if they travel through a nether portal from the overworld.

The developer wrote that we should take the problem up to you, I'm sorry that I'm not able to tell you what the problem is exactly but thats the quote from the developer:

"_It's a bug in Spigot to do with the PlayerTeleportEvent not always reporting the correct getTo() location in relation to nether portal teleportation. Take it up with them.

Specifically, getTo() is returning the location coordinates they're coming from in the overworld rather than their destination location coordinates._"