Why is Hamlet so furious at Laertes for his manner of mourning his sister in Act V, scene 2?
In Act V scene 2 Hamlet's fury has ameliorated significantly. This scene is the final scene where the two meet and Hamlet attempts reconciliation with Laertes. He asks for his forgiveness "That I have shot my arrow o'er the house/And hurt my brother." Laertes accepts his apology in "nature" but apparently still is the one who is furious, not Hamlet. He says,
"I am satisfied in nature,
Whose motive in this case should stir me most
To my revenge. But in my terms of honour
I stand aloof, and will no reconcilement..."
This quote suggests that Laertes has not yet felt vindicated for what Hamlet has done to his father ,Polonius, and his sister, Ophelia. At this point, Hamlet has already decided that "there is a divinity that shapes our ends, rough hew them as we will", so he has relinquished much of his fury into the hands of God. He knows he could die and tells Horatio that "the readiness is all" as he willingly goes into a battle that he knows full well could be his end.
I was always a bit sceptical about both Hamlet and Laertes at this point in the play. Apart from the possibilities mentioned above, which are perfectly valid, could it be that there is a bit of male competitiveness here even in something as non-competitive as grieving? Hamlet - and remember that among other things Hamlet is very much an actor - has seen Laertes's perhaps extravagant show of grief, suspects him of putting it on a bit and so, not to be outdone in the acting stakes, puts on his own show of emotion. As to what he is doing in the graveyard in the first place, that's another good question.
I'm guessing you mean Act5 Scene 1 (not 2).
Hamlet is lurking in a graveyard (for no understandable reason, but no matter) where there is about to be Ophelia's funeral. Laertes gets very angry with the priest who will not give her a full funeral because she killed herself. He starts shouting and jumps into Ophelia's grave to kiss his dead sister (ew...)
Hamlet, who has only now discovered that Ophelia is dead gets all shouty too and attacks Laertes and says he is even more upset.
I have always thought that Hamlet's attack is rather meaningless. I think it is just Hamlet's grief at Ophelia's death.