Why and when do Mac laptops start themselves when connected to power?
A Mac laptop will boot itself when power is connected if (a) the firmware power on time has passed since the machine was last running (i.e. if the machine was set to power on at 6 AM every morning and it has been 6 AM since it was last powered on) OR if the machine was on (not shutdown properly) before power was removed.

The concept is that if power is lost, a Mac should return to the state it was in.

If you want to ensure that the machine does NOT power itself on, 


In System Preferences -> Energy Saver ->Schedule..., remove any rule that states the machine should power on at a given time.
Shutdown the machine correctly by selecting "Shut down..." from the Apple menu and waiting until it is completely shut down.