Thanks for the replies. I agree with the posts that it is handler created due to incorrect release of the chasing pressure. I recently watched an experienced handler cue her horse to do inside and outside turns with the tiniest posture changes and then saw the same horse start kicking out, pinning its ears and threatening to strike when handled by someone learning the cues. The horse appeared very confused about what was wanted of it and expressed the resultant frustration with aggressive displacement behaviour. When the original trainer stepped back in the pen the horse went back to responding correctly without the aggression.
What I am really interested in from a research perspective is how frequent it is and the contexts in which it is likely to occur. Anyways am just at the very beginning of developing this project so appreciate the comments- they've given me a lot to go on with.