Generally, I’d just put this down to another bit of random weirdness, but as it happens there seems to actually be a reason behind the eruption of 2012 fervor around Mount Rtanj.
But an older layer of legend also claimed that the peak once held the tower of a powerful wizard, as tall as the mountain itself, within which a fabulous treasure lay concealed. Due in part to the peak’s propensity for being struck by lightning, legend also says it is surrounded by spirit lights. These, in turn, gave rise to the modern myth that it was a base for UFOs, thus yielding the alien pyramid story when all these tales merged into a newer, sillier one.
The idea that a mountain is the right location for a hidden treasure is an old one in Indo-European lore, going back to the idea of the Sleeping King beneath the mountain as well as the myth of the dragon, the serpentine monster that supposedly guards such treasures. Consider, for example, the stories of Beowulf or Sigurd, or, more to the point, the fact that the Serbian hero Prince Marko is one of the Sleeping Kings. This Marko was a historical figure, but his folklore counterpart incorporated older myths and legends from pre-Christian times. Thus, the Marko of folklore was a giant who did not die but either was sealed up in a cave atop a mountain or lived in a vast house concealed in a pit under the earth. It isn’t hard to see the alien pyramid within Mt. Rtanj as a variant of a supernatural underground palace like that of Marko (not to mention that of neighboring deity Zalmoxis), which allows those within to live on past the point of natural death.