Has there been some sort of change just prior to it happening?

I only mention this as I (barely) remember reading somewhere a story about a little girl (about 18mths) who pulled out her hair after her parents tried to get her to sleep in her cot all night (as she was previously sleeping with them in their bed).. The "expert" response was that she was doing it almost as a comforting thing and that they should "listen" to their child rather then feel pressured by family & friends to have her sleep in her cot etc..

Maybe your DD is doing it in response to something that has unsettled her? maybe lots of reassurance and distraction are what is needed...

I hope this helps