Ah, I see... And netherrack already gets mined really fast anyway, even when just using a stone pickaxe.
Heh, that dang lava. The pigmen are responsible. They act 'dumb' but behind the scenes you know they were waiting for you. As lava does not effect them, they hurry and watch where you mine and place strategic lava pockets ahead of you and lay in wait, watching. Then when you hop about on fire and die, they laugh behind your back.