Anything other than peacefully grazing or loafing around the paddock is potentially stressful to horses.
The threshold at which they become stressed and how quickly they chill out again can depend on their magnesium levels.
As magnesium plays an integral role in nerve relaxation, brain function and muscle release it becomes obvious how important it is. In fact magnesium has at least 350 different jobs to do in the body so is constantly used up and needs to be replenished daily - multiple times to be ideal but obviously not practical so at least once a day is minimum!
Travelling, having the teeth done, being started (or broken in), competing, racing or being sold and moved to a completely new environment all place the horse under considerable stress.
It works extremely well to double their dose the night before and the morning thereof any such activity.