Well, I'm going to add an option: crust-buster bombs.
That's right, ladies and gentlemen, we've created weapons capable of simultaneously creating a hole from the surface to the mantle and the pressure waves necessary to trigger an instantaneous volcanic eruption through that same hole! Thanks to our new handwavium technology, this awesome force can be yours for the mere price of...
(Terms and Conditions may apply, not guaranteed to work in areas with thicker-than-average crust such as existing hills and mountains.)
I'm sure you can find some sort of way to justify a world where people went crazy with the crust-busters and basically turned the entire planet into a series of volcanoes for a few months. Bam!!! No more flatlands.
If you want a more 'natural' explanation, I can't do better than Andrei.