Why does my central heating system comes on on its own accord?
If your system also heats your domestic hot water, it may be coming on to maintain the hot water temperature.   Depending on the setup, you may be getting some hot water circulation through the radiators when this happens.    Is the radiator system steam, or hot water?   If it's hot water, can you tell of one or more of the circulators come on when the boiler runs?