Have you been placing your phone on the computer? I have heard that magnets in the phone can trigger the shutdown. On my 17in 2011 macbook pro I set my phone on the bottom right corner and the computer goes to sleep, this is because there are magnets in the monitor and the computer detects the monitor's magnets and goes to sleep. This is how the computer detects the lid being closed. My friend once did this by accident somewhere else on his computer and it shut down. Make sure you are keeping magnets away from your computer.