Runaways happen both under saddle and in harness, and there are five reasons for them:
1. The horse is bolting and panicked, per my last post. This is always dangerous and particularly so with harness horses.
2. The horse startled and ran away from something briefly - for example, I've had a horse take me halfway across the arena jumping and bucking because a tree fell down in the woods on the other side of the street. It wasn't a true bolt because he got his composure back very quickly.
3. The horse is in pain or discomfort, for example from ill-fitting tack or a need to visit the equine dentist.
4. The horse is over-excited and wants to run, or thinks it's "time" to run for some reason. This is most often seen in retrained racehorses, who sometimes seem to forget they're not on the track any more (not all of them, mind). I was also once with a trail ride of 30 horses and somebody blew a hunting horn. Every horse that had followed hounds immediately tried to take off running. The horses that hadn't looked at them like they were crazy.
5. The horse doesn't want to work and is taking off as a way of avoiding a difficult exercise or intimidating its rider.
A good horseman eliminates number 3 before addressing 4 or 5. Number 4 is often best addressed by more work or more turnout, or sometimes by taking the horse out on the trail and running it out of their system. 5...well, I tend to put a horse that tries that to more work. I also know one horse who takes off if he's bored...if you work him hard, he stops doing it.
Regardless, being "run away" with is an occupational hazard of being a rider and trainer.