The Age article doesn't mention it, but according to some other reports they had left Bangkok with a non-operational APU.
So of the 3 "engines" on board they were down to 1, on a plane that was only 3 weeks old. You'd have to suspect that anyone would want to divert under those circumstances...
From
http://www.smh.com.au/articles/2007/...042982638.html
"Before we took off the captain explained we had an inoperative auxiliary power unit and that the left engine was compensating for this, running the power of the plane," Mr Messer said.
Presuming this is true you'd really have to question if they should ever have even taken off without a functioning APU - especially for an ETOPS flight?