I believe the Queen thinks that A. the death of his father caused Hamlet to go insane.
This is what she says in Act II:"I doubt it is no other but the main: His father’s death and our o'erhasty marriage"
This is what she says to Claudius, her ex husband's brother whom she married after king Hamlet's death. She clearly states that it is the king's death which drove Hamlet to madness, as well as the fact that she remarried only a couple of days after that happened. Polonius thinks Hamlet is mad because of Ophelia and the fact that she killed herself, but Hamlet never really cared much about Ophelia (especially after his father's death), whereas Gertrude thinks it is because of his father.
