I suspect they're an extreme form of "contrarian" that craves disagreeing with conventional beliefs to the point where it becomes part of their self-image/identity. Like other conspiracy theorists, they're obsessed with intricate rhetoric and arguments, but they rarely take the extra step of trying to prove something without obfuscation and without ulterior motivations.
This guy strapped himself onto a homemade rocket and lit a fuse. There are easier ways to prove/disprove the Earth is flat-- even if you don't buy the shadow-measurment and clock experiments. One could, for example, mount an expedition of to reach the edge of the Earth and take pictures. But that hasn't happened, ever. Or perhaps it did happen, and the effect was merely to quietly convert the very chagrined expedition team members from flat-earthers into round-earthers?