The electrical pathway is reversed; whether that be misplaced leads or an abnormal conduction system. If the wave is going up (positive), the pathway is moving towards the lead you are viewing (ie lead 2, left leg looking towards right shoulder)
Your electrodes basically form a camera and a backdrop. The positive electrode is at the lens and the backdrop is at the negative electrode.
If positive signals moves towards an electrode, it goes Up. (things moving towards a camera get bigger)
If positive signals moves away from an electrode, it does Down. (things moving away from a camera get smaller)
Likewise (this takes some brain wrangling at first):
If negative signals moves towards an electrode, it does Down.
If negative signals moves away from an electrode, it does Up.
So, if you have a predominantly negative complex (aka a qs or QS complex), one of two things is happening:
A positive signal is moving away from the electrode.
A negative signal is moving towards the electrode.
Since we know that ventricular depolarization results in a positive signal, this must mean that the signal is moving away from the positive electrode.
We call figuring out where the signal is going the Electrical Axis of the heart.