I'm 99% sure I used volume down button. To be honest I can't remember but it's the same on all my android devices, so I did it instinctively.
Maybe it's the app Micro CPU monitor? It has an option to show all cores seperately and also starts on boot.
So it's very likely that apps that access the processor can cause reboots.
BTW some fun facts taken from downloads of this App:
- The Phone is called P880 3x instead of 4x on the Play Store
- Half of the phones seem to be in used in HongKong and Taiwan
- 10 percent in Italy
- 5 percent China, 5 percent Germany, 5 percent Russia, 5 percent Thailand
- USA and Great Britain didn't even make the list.
Just my 2 cents.