How to identify the cause of a restart on MacBook Air via Console app in OS X Yosemite
Using the console


Open Console app (by default located in Macintosh HD⁩ ▸ ⁨Applications⁩ ▸ ⁨Utilities⁩)
Select the system.log file in the left pane
Conduct a search for Shutdown Cause
View the filtered output


See Yosemite example below:



NOTE: For more context, you may want to search for Shutdown only to see some of the other log entries and not just the Shutdown cause.