you're body is realising that its a foreign object and therefore becomes infected and becomes pus filled (this is our bodies way of cleaning a wound). Its trying to get rid of the piercing, best idea is to use salt water and paw paw ointment to try and help the infection or take it out. surface piercings usually grow out anyway.
Your body pushes them out, bit by bit to the point they just fall out randomly one day. Thats what happened my eyebrow :P Its better to take them out when this happens, it leaves a scar. All surface piercing do this, but the length of time it takes varies from person to person.
hip piercings are surface piercings and all surface piercings growout sooner or later. rejecting however means that your body cant handle them being in there so they push them out and the wounds wont heal with the jewlerey in so you need to remove them and let them heal over.
I'm guessing that its when your body is rejecting the piercing. I know someone who got an eyebrow piercing and her body rejected it, leaving a massive lump and not so attractive scar on her face.
I'm sure it means that you grow out of them so that you have to take them out. I'm also sure that they only last a few months anyway.
may be remove it because your body is not allowing to wear it..