I am not trying to pick on you. I just know that avoidance of a problem oftentimes ends up with the horse escalating the behavior. It can go from biting (which tying short would help) to kicking to tearing a trailer apart to crippling themselves. I've just seen it happen too many times and would prefer to actually change the behavior.
Horses do not need 'good' reasons for anything they do. Sometimes they have a reason -- like an incident years ago and an unwanted behavior was the horse's answer to that reason. From that day on, they are 'creatures of habit' and the only reason they needed was that they did it before. This is, by far and away, the main 'reason' most horses do most things they do that make little sense to us.
We also have to understand the difference between a 'reason' and an 'excuse'. Even if horses have a reason, we should not let it become an excuse.
Their 'reasoning' and their 'cause and effect' has to be kept pretty basic and simple. Most people waaay 'over-think' what a horse is thinking.
I have had horses that were bitten by one other horse one time under saddle and I had to go through a whole lot of this type of 'behavior modification' because from that day on, they fired (kicked) at any horse approaching them from the rear. Many times people do not even know or remember the first incident that got a bad habit started, but you can bet the horse does. You just have to do something to 'break the cycle'.
No, I would not tell your frail 79 year old mother to do this. I would expect you, or some other able bodied person, to go help her and 'set the horse up' and 'fix' the problem for her. I am very crippled and I still fix these kinds of problems for people that come here with a horse. Then, I make sure they know how to safely do it themselves. The last thing your elderly mother needs is to be hauling this horse and have it have a 'come apart' on the road.
You simply put one regular lead-rope on a nylon halter. You put on a second lead-rope with a chain on the horse. Throw that one over his back; load the horse; treat him just like you ordinarily would; tie him; then reach in the trailer and take hold of the stud rope; hold it outside of the trailer; and wait for the horse to misbehave. When he does, you jerk his chain. I would load 2 or 3 different horses behind him. I would make sure he know why he was punished. This is a very quick and very simple 'cause and effect' that they all understand.
I have ridden in the back of a puck-up on on a flatbed several times holding a rope attached to a horse that we knew was going to start kicking and fighting a trailer or his neighbor. It has always changed a horse's behavior.