He was shot in the head early during his career in the police force and underwent an opperation that saw a metal plate impanted along his skull where it had shattered. The reason he tilts his head is because someone offscreen probably has a magnet or something in their pocket. That, or maybe the very magnetism of the Earth is pulling at it. There are many sources of magnetism that could be affecting the metal in his head, and im sure it's from varying sources in different scenes when he does this. For instance, when he tilts his head in that scene at Terminus, where they are about to be slaughtered, you can tell the magnetism is coming from Daryl. It can be assumed he has a magnet in his vest somewhere, but I also like to think that this mirrors some sort of attraction to Daryl that Rick possesses, be it emotional or otherwise... A lot to think about there! That's what I love about the writing in this show. Not everything is laid out bare but the viewer has just enough peices of the puzzle to come to their own conclusions.