DD was not posterior and my labour stalled at 7cm and was even told by the midwife that she thought I was going backwards! This was in a Birth Centre and I was very comfortable, no fear, nothing changed etc to 'cause' it to happpen. I was transferred to a hosp and told by diff obs and drs that it is not uncommon in a first labour for this to happen and most unusual in subsequent labours and I have even read into it online and saw it in the very general What the Expect when you are Expecting book. So I can't answer the original qu- 'What causes it to happen' but just wanted to add my bit and that it is not unusual!