So the problem has nothing to do with the Arduino, it's the port expander.
Not necessarily. There is a similar complaint in the Motors section about an Arduino Uno that is fiddling with the PWM pins while the bootloader runs. There is nothing blatantly wrong in Optiboot but westfw found a very subtle bug involving the R0 (or R1) register initialization. It is possible that @gladiatoreAde is effected by that bug (or another very subtle bug).
I obtain the same problem if i connect an lcd display I2C to arduino...it activate baklight for some seconds..
If the back-light is being controlled by a PWM pin then @gladiatoreAde's problem may be identical to the one reported in Motors.
@gladiatoreAde: Which Arduino are you using?