While I think you're right that competition is much more of an indicator, you cannot discount oil prices:

"As Richard Branson, chairman of the Virgin Group told CNN Business Traveller on this month's show, Virgin Atlantic's annual fuel bill has risen from £250 million three years ago, to £1.4 billion this year.

The only way to survive the increase, he says, is to pass the cost onto the consumer."

http://www.cnn.com/2008/BUSINESS/06/...ces/index.html