When pushing back and having a direct tailwind from behind the engines can cause that smell as previously mentioned.
Ive found setting the extract push button to override- closing the avionics inlet and extract valves- once the smell is first perceived eliminates this problem. In the cockpit at least. Not sure if it elimintes the smell from the cabin.
After second engine start. PF does normal flow and sets the override push button back to normal. Inform the other guy as well before you do this so neither of you forget!