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.
I've been able to produce a reboot myself: When switching from eco mode to quad-core with a tegra game playing in background.
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.