Answer:
The fight began because of Gregory when he asked Sampson to provoke two Montague servants who were approaching towards them
Step-by-step explanation:
The act I begins with a fight when Sampson bites the thumb of Montagues which was considered as an act of insult and hence after the verbal confrontation the fight began.
Basically, the root cause of this fight was the Gregory who asked Sampson to provoke two Montague servants who were approaching towards them.