This might be verging on "a wizard did it", but I have been toying with the idea that deviants from a really advanced civilization (verging on post-scarcity from our point of view) might go off to play great man among the primitives. As a slightly more sane reason for an alien invasion. Why invade earth? To have people look up to you as powerful and important instead of near-janitor in status. And they would mostly invade British East India Company style. Say they are here for trade and actually start inserting themselves into the economy so as to make themselves monopolists and/or monopsonists in certain sectors of the economy and then taking political power from that.