I used to get a similar sensation just when doing front kicks, but in the last few months it's started happening with crescent kicks as well, and more recently even with leg raises and situps. Basically anything that involves bringing my right knee towards my chest (or my chest towards my knee like with the situps) will make it feel like something has moved out of place in the front of my hip joint, and sometimes I have to shake my leg out to make it "click" and go back to normal. Ocassionally this click is loud enough for everyone in the room to hear.
Initially I assumed the problem was caused by tight muscles in my hip, but yesterday a biology student suggested I may have torn the cartilage in my hip joint. I've looked around online and the symptoms of a hip labrum tear seem to describe exactly what has happened to my hip, especially on this page where it mentions the "catching" sensation:http://www.mayoclinic.com/health/hip-labral-tear/DS00920
I'm planning to go to my doctor asap and get this checked out.