Horses are a 'herd animal' by nature. You have to actually 'train' them to listen to their rider instead of place 'being with their herd' as first and foremost in their minds.
It is not related to age or to training level under other circumstances. They may be high level show horses and still get crazy when left behind on a group ride out on the trail. When they head out on the trail with other horses, unless they have been trained to still be a submissive member of your 'herd of two', they instantly become a happy member of the new herd they are now with.
We start out schooling horses with two horses and riders. We play the 'leapfrog game'. This is where we start out together and then one horse goes ahead out of sight. The horse left behind is trotted in circles, trotted back the other direction or gotten off of and tied to a tree until they settle down.
When they finally walk quietly, they are trotted forward until they pass the first horse and are expected to go on past it quietly. This game is 'played' for as many days as it takes for them to learn that life goes on when they are separated from their current herd.
We start doing this when we first start going out on the trail with a green horse. The ones that we are training now, went out on the trail about their 4th or 5th ride. Their very first ride started with 2 horses and they were ridden off from the other horse on that very first ride.
We have found that the sooner you deal with their 'herd mentality', the easier it is to get good trail manners. Good trail manners (all necessary for any good trail horse) includes the willingness of a horse to stay behind when all the other horses disappear ahead of them.
When I ride a horse as a 'guide horse' on about 3 or 4 trail rides, I know I have to take him out with a colt and do the leap-frog thing or at least do the 'ride off in different directions' thing because he will try to whinny and will not be happy about going on or being left behind. It is just part of the ongoing training that every horse needs to become and stay a good trail horse.
The two main things that every horse is 'hard-wired' for are:
1) Their dependance on their herd for safety.
2) Their natural response to danger or perceived danger is 'flight'.
You spend much of every horse's riding life, training them to listen to their rider instead of natural herd and flight instincts. The sooner you start on these with a green horse, the sooner you get a dependable 'fearless' trail horse.