1. Gertrude has summoned Hamlet to her chamber because she wanted to see why he was acting strangely lately and if he is experiencing any problems. She called him in order to talk to him and determine the cause of his weird behavior. She admonishes him because of his erratic behavior and disrespect towards his uncle/stepfather, whereas he scorns her for disrespecting her late husband by remarrying so quickly.
2. Hamlet is even crazier in this scene, thinking that his mother has betrayed both his father and him by marrying his father's own brother, only a couple of days after King Hamlet's death. He isn't thinking clearly, he is paranoid, angry, which when all of these emotions accumulated resulted in him inadvertently killing Polonius, who was hiding in Gertrude's chambers, eavesdropping on their conversation.
