Download wakelock detectorWakelock Detector from the google play store, ifGoogle Play Store. If it happens again, open wakelock detectorWakelock Detector and it will show you cpuCPU and partial wakelocks (will only show partial wakelocks if you enable advanced mode), if. If the largest cpu wakelcokCPU wakelock is "PowerManagerServiceWakelock", this means the largest wakelockwake lock is the combination of all the items listed in partial wakelcockswakelocks.
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I would suggest (if it happens again) looking for anything over 7% and doing a web search for the wakelock name, and seeing what you can find.
If it doesn't happen again, problem solved.
I would also recommend you don't kill background processes, instead install greenifyGreenify (play storePlay Store) which blocks the processes you select from starting.
Other than this, we can't tell you the reason due to androidsAndroid's default battery diagnostics not being very good.
It is most probably the software, if you download Nova battery tester and run it, it will tell you your battery capacity, and you can then compare this to what it should be to verify it isn't a hardware fault.