How does Warwick Schiller's attachment theory work apply to horses that won't trailer load away from their herd?

Warwick Schiller's incorporation of attachment theory — borrowed from developmental psychology — into horse training has specific application for horses that load fine at home but refuse to trailer away from their herd or familiar environment. Schiller's framework identifies these horses as having what he calls an insecure attachment, meaning they have not developed genuine confidence in the human as a safe base, and as a result they fall apart when the security of the herd is removed. The trailer-away-from-herd refusal is one of the clearest expressions of insecure attachment in horses, because what's actually happening is not that the horse is afraid of the trailer — it was fine with the trailer at home — but that the horse is afraid of being without its herd, and the trailer is the vehicle of that separation. The horse's resistance at the trailer is really a protest at the separation itself. Schiller's solution is not to work on trailer loading directly but to work on the underlying relationship — specifically, to become someone the horse genuinely wants to be near. He talks about doing low-pressure, low-demand activities with the horse that build positive association with the human's presence: grooming, hand grazing, standing together quietly, doing liberty work where the horse chooses to stay rather than being held. Over time, the horse develops what he describes as the human as a safe base — the confidence that being with the human is safe enough to tolerate separation from the herd. Horses that have developed this genuine relationship with their handler will walk into a trailer away from their herd because the attachment to the human provides enough security to override the attachment to the herd. Schiller notes that many people try to solve this problem through desensitization and pressure at the trailer without ever addressing the underlying relationship deficit, and those horses load under duress but never develop genuine confidence.