Kernel: "Previous shutdown cause: -62" I have a late 2013 27" iMac running  Yosemite, and it randomly shuts off every now and then. I tried everything Reset SMC. Diagnostics, reformat Macintosh HD but still occurs and running out of options.
If this is new behavior, blame it on Yosemite, file a report, and update whenever available. I'm on Beta 3 or 4, and am just now occasionally logged out at random. Earlier glitches have been fixed, as will this one.