So, I take it this is what would have happened had Fizzle not had her horn damaged.

@Background Pony Number 17  
I think the only way magic works at all in Equestria is with mana being a constant within the universe, sort of like dark matter. In a short explanation, a unicorn or alicorn’s horn allows them to manipulate the mana directly within space and time, which in turn allows extraordinary things to happen. This manipulation can be anything from simple levitation to time or reality warping. We’ve seen that it allows atomic rebinding and restructuring to turn one object into another, or transporting an object out of negative space and reconstituting it into another space.
 
It’s really complicated, but I’d put my money on everything just being how adept the magic user is in terms of their control. In this explanation, Fizzle COULD light the gunpowder on fire, but instead is just manipulating it’s position within space.
 
And she’s not even doing that. She’s actually just handling the bag.

I have to wonder if handling gunpowder with magic is safe.

You know, I honestly thought those were coffee grounds.