Because of its wide elliptic orbit it enters into our solar system per 3.600 years. While this passage it can cause some natural disasters like volcanic eruptions, earthquakes and polar shifts. Some historians claim the sunk of Atlantis and Mu continents into the ocean has resulted during these passages. Apart from Nibiru, today "Elenin" and "Comet Ison" are the ones which may enter into the solar system and possibly hit the earth.
Astronomers on the other hand reject the theory of Nibiru which is claimed to repeat its passage around 2900 AD. They claim such a big object even if out of the solar system could be seen with naked eye or it should had a kind of effect on the orbits of the planets so far.