Watch to see if he does it when grazing. If so try a few simple tests like making him flex down to the ground to eat a carrot. See if he does it then. Report at what point it starts and stops. Try the neck flexed at different angles. It could be a muscular/nerve response in the neck or back. It might be neither. Do some more investigations and talk to your vet. Best of luck.
Raise his bowl up when he eats and see if he does it.