Even in the case of overpopulation, an alien invasion by a species capable of interstellar travel doesn't make much sense, for several reasons, of which I'll only list 4 cause I'm lazy:
1-By all we know of exoplanets, there's likely huge numbers of habitable worlds out there with no technologically advanced species on it. One would think that a species capable of interstellar travel would prefer to target a world without having to worry about a war seasoned species capable of limited space travel and possessing nukes;
2-Any species capable of interstellar travel en masse has likely has solved all the problems that would force it to raid a planet like earth for resources. Also, there are far better sources of pretty much any earthly resource out there in space;
3-Any species capable of interstellar travel likely has weapons at its disposal that would allow them to wipe out all of humanity. There would be no invasion, just a single, calculated strike;
4-Its highly unlikely that the aliens would invade out of interest in humanity itself, as we don't really have much to offer them, and our biology isn't exactly extraordinary either;
The only somewhat believable reason for an alien species to actualy bother to invade are religious or cultural reasons. Maybe their god demand them to invade worlds inhabited by advanced civilizations for whatever reason, or their culture is built around probing anal cavities or somesuch.