MacBook boots automatically when opening lid. Feature or problem?

When my 13" Retina MacBook running Yosemite is powered off and I open the lid, it immediately starts booting up... without pressing the power button. Is this a new feature (maybe part of Yosemite) or is my MacBook misbehaving?

If it's a feature, I'm not necessarily sure that I dislike it... but I definitely find it jarring. Can I disable it?