This can happen if the starter didn't give enough kick for the engine to start rolling more, the button won't pop away, so when you click on the cartrige reloader, it will instantly ignite next cartridge, you must check if it pops up before reloading the cartridge.
In the real spit the starter button is a click and hold button (for what i know it was in mkI, not sure about mkII), i read it will be changed in the next update.
Curious, if the pressed starter button would really ignite a reloaded cartridge, or one would need to depress it and press once more.