Both individuals are driven by the emotion of Ophelia's death. Laertes jumps into Ophelia's grave in grief heightened by the churlish priest's bare funeral rites. Hamlet sees Laertes as over-acting his grief. Hamlet believes that his own grief exceeds that of Laertes. They both grapple briefly until pulled apart. It is a director's choice whether Hamlet leaps into the grave with Laertes or Laertes jumps out of the grave and lunges at Hamlet. In any event Laetes does manage to get his hand on Hamlet's throat. At this point one wonders why Laertes doesn't produce a blade of some sort and cut Hamlet's throat as he had bragged about in the previous scene. I think Laertes's grief is taken over by the sight of Hamlet and the emotion of revenge begins to consume him. The king certainly thinks so. He urges Laertes to strengthen his patience, reminding him of their plan. Just like Hamlet in the prayer scene, Laertes delays his revenge for a seemingly better plan.
But Hamlet sees something more in Laertes actions. It is not just grief for his sister that has Laertes attack Hamlet. He questions: "What is the reason that you use me thus? I loved you ever." It never occurs to Hamlet at least not until the next scene that Laertes holds animosity toward Hamlet for the death of Polonius.