Previous shutdown cause "5"

[Ram171]

I normally put my MBP to sleep, but recently I've been shutting down. The reason for this is because of the long periods of time in between use. Today when i power on, the log in screen appears, i log on and quickly i get some "apple debugger" black letters and the MBP prompts me to log in again..all within 3 seconds..very fast. All i got out of that was "apple debugger". Once in, I go into terminal and see crash. and in console i get a "previous shutdown cause: 5". What does this mean? As far as I know i dont just press the power button and close the lid. I press the apple button on the top left and select "shut down". Whats wrong???