Why do Sampson and Gregory fight with Montague's men in Act 1, Scene 1 of Romeo and Juliet

The quarrel in Act I scene I starts, initially, because
Samson decides to “bite his thumb” at Abram; this gesture is synonymous to the middle
finger.   The Capulets have drawn their weapons are waiting for the Montagues to make
the first move.  Sampson says to Gregory, “Nay, as they dare.   I will bite my thumb at
them, which is disgrace to them if they bear it”(1.1.44-45).   By insulting the
Montagues, Sampson hopes to provoke them into a fight which would allow him to later say
the Montagues intiated the brawl.   The examination of the servants at the beginning of
the play provides the reader the feeling that this “ancient grudge” exists throughout
each family’s household – from top to bottom.