Orlando has many grievance, and they are against his eldest brother Oliver. Their father, Sir Roland de Boys, specified in his will that the eldest brother Oliver take care of the raising and educating of the two younger, Jacques and Orlando. Oliver fulfilled his commission regarding Jacques, who was carefully educated as a gentleman, but failed in his responsibilities to Orlando--possibly because he resented the expenditure of more of their inheritance.
While Jacques went to schools, Orland was left to live in the stables of the family manor:
he keeps me rustically at home,
differs not from the stalling of an ox?
He complains to Adam, his servant and old family domestic retainer, that Oliver spends money on "riders dearly hired" to train his horses, yet Orlando gets nothing from "him but growth." Encounters with Oliver even seem to rob Orlando of his good nature because Oliver treats and trains him "like a peasant."
These are the grievances Orlando has plus the additional one that, after having won the wrestling match against Charles and incurring the further enmity of Oliver along with the fresh enmity of Duke Frederick, Oliver plans to burn down the dwelling that has housed Orlando.
It is for this murderous new twist their brotherly love has taken that Adam suggests he flee to the safety of Arden Forest. The final grievance accrues to this because in fleeing for his safety, he loses Rosalind (or so he thinks).