There have been many examples of this throughout the long history of man's relationship with the bulls and indeed, at present you can find similiar kinds of bull-running being held in several towns around Spanish State. The basic idea is that it is a way of getting the bulls into the bull-ring from whatever kind of corral or enclosure they are being kept in.
The different circumstances in each particular place means that there are some change in details but not in the essential spirit of the act. Throughout its long history, there have been different methods employed to bring the bulls up to the bull ring. There was a time when the bulls were not lead by runners. But at some uncertain date it seems that the butchers guild were responsible for bringing in the bulls (and for this reason they had the Santo Domingo stretch for themselves alone) and they began to accompany the bull minders or drovers in bringing the bulls up from the pens in Santo Domingo to the bull ring.
As time passed the event became more and more popular and some people began to run in front of the bulls and not behind them, as the drovers do. In 1852, a new bull-ring was built and a new route - the actual route being used up to the present - was decided on and which ignored the traditional Chapitela street.
The run soon became much shorter also, because as from 1899, it was decided to bring the bulls up to a small corral in Santo Domingo street the night before they fight in the ring. So that is why there is an "encierrillo" each night before the morning run, when the bulls are moved from their enclosure on the far side of the river, up to the small corral at the bottom of Santo Domingo street where they will spend the night before making the morning run up to the bull ring.
FIND ALL THE DATA ABOUT THE RUNNING OF THE BULLS FROM 1980 TO NOW: Running of the Bulls Database