My IUD partially expelled right around 6 months. I actually didn't feel it at all, or experience any complications beforehand. It actually came on pretty suddenly. I discovered it was expelling because my husband said that he felt something kind of hard and poky toward the end of sex >.> (sorry if that was tmi) I immediately went into the bathroom and tried to feel for it. The strings were much longer and lower, and I could feel the tip of the IUD itself up by my cervix, where you would normally just be able to feel the ends of the strings. I immediately called my GYN, who told me I'd have to get it replaced, but I didn't have to get the old one taken out right away. It wasn't hurting anything to leave it how it was until I could get an appointment -- the only thing was that if I had sex again in the meantime (which I did not), it would be unprotected. My appointment to get it replaced was less than a week later, and I couldn't really feel it during that time. I've had my new IUD now for about 5 months, and fingers crossed so far I haven't had any problems with it.
As far as the possible causes of expulsion... I have not had any kids before, which I consider the main contributing factor. From what I understand about IUD's, the expulsion rate is higher (as low as 5%, as high as 10% depending on the source? I don't know the actual figure) among women who have not had children. I was just coming off of my period when my IUD expelled, and I understand you're also more likely to expel around that time.
The other thing was that I had just started using a menstrual cup before my IUD expelled to help manage the heavier flow that resulted from getting the IUD. I asked my GYN about it when I called her, and she said that it was probably just a coincidence, and almost all of the reading I did afterward said that there's no correlation between IUD expulsion and what you use for menstrual hygiene. I still continue using my menstrual cup (I don't think I could go back now!), but I do still worry every month if it will dislodge my IUD. But, so far, so good.