When you say shut off, how did it shut off exactly? Like a clean shut down "Windows is shutting down..." or like a power outage? From what you describe I guess you must be reffering to the second one. Well in this case, this could be a lot of things...
- Could be a defective battery. Not because it says 100% means it is. I saw some batteries saying 100%, and when you unplug them they suddenly drop to 5%, only to fail a few seconds later...
- You could have accidently pressed the power button, either on the laptop or in Windows while carrying it.
- Sometimes PCs and laptops will automatically shut down because of overheating. Did you check your temps? CPUID Hardware Monitor is a good software for this.
- Could be any other kind of intermittent hardware issue, bad CPU? bad motherboard? bad GPU? bad power supply? bad memory?
Really hard to tell you what happened exactly...