Some Odd Behaviour - Any Ideas?

I visited Old Moor the RSPB reserve near Barnsley this weekend. On what was a quiet day I came across a family of willow warblers hidden away in the bushes with the parents struggling to keep up with feeding their greedy brood. I struggled to get shots as they were deep in the undergrowth but after awhile they popped out in the open. One youngster sat with a parent who then flew off to get more snacks and the youngster suddenley stretched out and then lowered itself onto the bramble and seemed to play dead. It remained motionless for a couple of minutes before popping up and then fluttered off into the bush. I looked around for predators but saw none and whilst this one played dead its siblings were hopping around in the background. Has anyone noticed this before or have any explanation for this odd behaviour? I'd love to hear your thoughts? The photo's below show what happened.