Once it gets to reserve power, the power management system will determine when there is not enough power to continue to use the system in a stable manner and forcibly sleep the system. This is completely normal behavior. If you'd like to reset the power management system, reset your PRAM by powering the system off, power it back on and press and hold Command-Option-P-R. Wait for the system to chime 3 times and then release.
I see a few people recommending this, but they're all wrong because it's not how you reset the PMU. PRAM has nothing to do with power management.
PRAM contains these:
Status of AppleTalk
Serial Port Configuration and Port definition
Alarm clock setting
Serial printer location
Attention (beep) sound
Caret blink time (insertion point rate)
Mouse scaling (mouse speed)
Menu blink count
No mention of power management there...!
PMU is reset like this:
MacBook, MacBook Pro
If the computer is on, turn it off.
- Disconnect the AC Adapter and remove the computer's battery.
- Press and hold down the power button for 5 seconds and then release the button.
- Reconnect the battery and AC Adapter.
- Press the Power button to restart the computer.
Now if the power management unit really is messed up, after doing this, you'll see that the amount of power remaining has possibly changed.