Camera starts on boot after L upgrade - Nexus 10
When you unlock how are you swiping?  

If you swipe to the right, that will open the dialer after you input your pin.

If you swipe to the left, it will open the camera after you input your pin.  

If you swipe up, it goes straight to the home screen after you input your pin.

(https://productforums.google.com/forum/#!topic/nexus/TkooKgxhgmg)