Thanks @fbara for suggestion to add more details
Device: MacBook Pro (Retina, 15-inch, Mid 2014)
OS: 10.11.6 (15G1004)
pmset -g log from one of the times it occurred
UUID: (null) 2016-09-07 20:15:11 -0700 Start powerd process is started 2016-09-07 20:15:11 -0700
Summary- [System: No Assertions] Using Batt(Charge: 81) 2016-09-07 20:15:11 -0700 Assertions Kernel Idle sleep preventers: IODisplayWrangler 2016-09-07 20:15:43 -0700 Assertions PID 104(hidd) Created UserIsActive "com.apple.iohideventsystem.queue.tickle" 00:00:00 id:0x0x9000001f5 [System: PrevIdle DeclUser kDisp] 2016-09-07 20:16:52 -0700 Assertions PID 261(Google Chrome) Created NoDisplaySleepAssertion "Playing video" 00:00:00 id:0x0x500000223 [System: PrevIdle PrevDisp DeclUser kDisp]
syslog | grep -i "shutdown cause"
and post the results. You may also want to run Apple Hardware Test (AHT). Hold the D key while booting from a powered off state with the AC adapter connected. This will help you determine if you have a hardware issue