What motivates such characters Bartollo, Gabriel, Maria, Leonardo, Damian etc to leave town or else ultimately to leave & then return back to Mexico ?

In the days leading up to the Mexican Revolution against Porfirio Diaz, all the characters who occupy this small Mexican town sense its limitations as well as many of those of Mexican society more generally. Those who leave and sometimes return all have different motivations.
Damián Limón is distinct because he went to the United States to seek his fortune. When he returns home, lured by his inheritance, a heart attack kills his father. Then Damián himself kills a man but, after being lightly punished, also runs off to join the revolutionaries.
Gabriel, the church bell ringer, leaves in shame because his passion for Victoria causes him to overdo it in ringing the bells. María feels confined by the ordinary town. While she longs to visit the city and have a romantic life, she ultimately joins the revolutionaries and runs away with Lucas’s widow.