I just saw that DL 91 that operates PDX-NRT is returning to PDX after about 5.5 hours; does anyone know what happened?

It was about 2.5 hours en-route when it turned around? Anybody have any theories? It doesn't seem like a decompression, they stayed at alt. most of the time, but I can't rule out anything else.

Here's the link to flightaware:
http://flightaware.com/live/flight/DAL91