The most obvious cause of gagging is when something an "unusually large object," for example is physically hitting one of the trigger spots in your mouth or throat.
Anyone who expects this is frankly undeserving of a blow job in the first place.
Can You Choke your Penis
So if you've ever gagged on something like your toothbrush or a penis, if we're gonna stick to the theme here and were like, "Why is this happening, this thing is not anywhere near the back of my throat," it's probably just hitting a different sensitive spot in the back of your mouth real estate.
Description: Almost always this is an incredibly helpful mechanism — every time I make it through dinner without choking on a piece of bread, I figure my life is going pretty well.