Hurricanes backtracking and looping seem to be quite common in the Atlantic (Nadine, Jeanne, Alberto, Dennis), and West Pacific (Ernie, Nari, Fung Wong, Parma, Roke) but not so much in the East Pacific (where the tracks are way more linear, with much less latitudinal change).
In general, tropical cyclones are steered by the global wind field. The environmental wind field (winds that surround a hurricane) guides a cyclone along its path. Hurricanes are steered predominantly by winds in the upper atmosphere, especially winds from about 3000 to 10000 meters above sea level. High-pressure and low-pressure systems often cause cyclones to divert from their initially east-to-west movement. When a high- or low-pressure system is located to the west and extends far enough latitudinally, storms are blocked and a change in direction may occur.
In the case of Tropical Cyclone Nathan, there seems to have been a stationary high over southwestern Australia on 12 March 2015 that connected with another high to the north and prevented the movement of the cyclone. The presence of Tropical Cyclones Olwyn and Pam probably influenced the change in path.
Nathan started to move north as Pam was moving south (13 March) and then went east without that much obstruction (14-15 March). As the high pressure to the south moved east, there was a clear path for Nathan to continue moving west (18-March).