I have been having the same issue with my MBP waking up and the computer has rebooted.
However, after reading through this thread and doing some research I believe my issue wasn't that it was rebooting but it was logging me out. Sure enough, looking through my Security & Privacy settings (under Advanced), it was set to Log out after 60 minutes of inactivity. A log out will close all applications and when you wake it up, present you with the login, which could seem like a reboot.
I'm going to test this tonight but I'm pretty sure this is what I was experiencing. YMMV
However, after reading through this thread and doing some research I believe my issue wasn't that it was rebooting but it was logging me out. Sure enough, looking through my Security & Privacy settings (under Advanced), it was set to Log out after 60 minutes of inactivity. A log out will close all applications and when you wake it up, present you with the login, which could seem like a reboot.
I'm going to test this tonight but I'm pretty sure this is what I was experiencing. YMMV