Definately looks like a loss of shielding gas to me.
Would have needed to be quite draughty to have a total loss of shielding gas, perhaps you need to up the gas flow rate a little
Definately make sure that you protect the welding area from draughts tho.
Can't tell by the picture but if that is 3mm material being welded, personaly I would up the welding current up a little ( assuming you are using a 150 amp mig )
