Hi alawax,
I agree with SW1's diagnosis in post #4.
During the first automatic engine start attempt, only 1 igniter is supplied, but fuel is being supplied to every combustor. It takes some time for the flame to propagate around to all the combustors. During that time, the fuel that has not been ignited, is exhausted into the atmosphere as vaporised fuel and is very smelly.
As you push back into the smelly air, the Avionics Ventilation system Blower fan sucks the air in and blows it around the avionics equipment bay. Hence you get to smell it first.
see FCOM DSC-21-30-20 Avionics Ventilation, Normal Operation, Open-Circuit Configuration.