Probably a combination of 2 & 3 along with weird natural phenomena + the fact that human brains run on error prone wetware, are full of hacks, and make shit up all the time...

In short, given prevalence of aliens in popular culture + brains being predictive machines + existence of things that are hard to explain = it would be far more surprising if there weren't people claiming aliens.

Note 1: actual aliens would be really exciting, but Occam's Razor...