I'm a big proponent of the "too many bored and trained military guys" theory. The tipping point was Spain funding expeditions out into the new world because they had amassed a military complex and were teeming with young guys who wanted to shoot guns somewhere. They had a ton of guys left over from the reconquista.

Why they got to the America first probably is down to geography, at least on this one. The most advantageous ocean currents facilitated back and forth travel to Europe better than from anywhere else. Then they discovered the wind currents to Africa.

that's just how they got the advantage in the first place. Why did they do it? Internal contradictions within feudalism that required expansion from the mercantile class. Stuff like that. The burgeoning capitalist class was feeling the restrictions of feudal land management and something was going to burst eventually. Europe just happened to be in a position where that capital expansion was done through domination of the ocean.