Why did a container of oatmeal in my cupboard become overrun by mealworms?

It was SO gross. Weirdly, I had another container in a separate cupboard that never showed any such problems. So was it just a fluke? How does this even happen?

Might be too much humidity in that particular cupboard.
I suggest you buy some tupperware to store your food.
Real brand name tupperware, they're the best.

The eggs are already in it when you buy it. Unfortunately that can't be avoided. So when you come home from the store put it for three days in the freezer. That will kills the eggs. The same works for any kind of flour but that container has to first be wrapped in plastic. I just store things like oatmeal and flour in the freezer, it works really great, it's always fresh. If you decide to bake a cake or a bread you need to get the flour or oatmeal back to room temperature. To do that put the desired amount into a bowl and stick it for a while into the oven without turning it on.

As fantagirl said, the eggs were in there the whole time, I know with flour you can nuke the container and that will kill any weavil eggs that might be in there without changing the flour at all, but I don't know if microwaving oatmeal would cook it or not.
ive it a try anyway whats the worst that can happen?

The eggs were already there but there's actually nothing wrong with eating them. They are a good source of protein and it's only Western cultures that revile the practice of eating insects, worms, and grubs. They considered to be delicious just about everywhere else in the world.

The eggs were in there already when you bought it. As in, you were eating them. Sorry.