MBA late 2014 with 10.10.2. When powering up the MBA from the powered off state, it would seem to go to shutdown state again when I don't login after some time. If I left it for a while, I have to press the power button again to start it back up. Once I logged in, it stays on no problem. When this is happening, the laptop is connected to the AC adapter, and the light is green. All my other Win laptops will stay on once you power it on. It's my first mac, so I don't know if this is normal for a Mac laptop.