There were probably five major events that led to Brutus jumping on his own sword:
1. The suicide of Portia. Although he treats this without much emotion (Why, farewell, Portia. We must die, Messala), it's bound to be on his mind during the final battles.
2. The suicide of Cassius. This was his good friend who started the conspiracy and stood beside Brutus throughout all of his poor decisions. Since Cassius was in charge of the other half of their army, this would have been a big military blow to their morale as well. ("I owe moe tears to this dead man than you shall see me pay")
3. The subsequent death of Titanius - found at the same time as Cassius.
4. The multiple appearances of Caesar's ghost. Brutus had been questioning his own actions since the killing of JC. Seeing JC's ghost for the second time, really unnerved him (O Julius Caesar, thou art mighty yet! Thy spirit walks abroad, and turns our swords in our own proper entrails).
5. Finally, the fact that's he's in trouble no matter what happens. Antony and Octavious both have armies that are closing in around him, and his men are fleeing as fast as they can. His options are limited at this point. He chooses suicide as a method of preserving his own honor and dignity. Strato (who held the sword) says "Brutus only overcame himself, and no man else hath ho nor by his death."
If you put all of that together, Brutus probably felt pretty backed into a corner. It was an unexpected and ironic occurence, though, considering Brutus's speech in the beginning of act five concerning suicide: "I do find it cowardly and vile".
Brutus meets his end in Act V. He had been one of the leading conspirators of the plot to kill Caesar, but still, Antony calls Brutus "the noblest Roman of them all." According to the analysis at eNotes, excerpted here, Brutus was "the only one of the conspirators who killed Caesar out of love of Rome and the republic, not for personal reasons."
Further, "Brutus has fallen upon the sword after a lost battle between his forces and those of Antony and Octavius. Brutus had from the start been hesitant to join the conspirators who sought to remove the dictator Caesar from power. Once convinced that the republic was in danger of being ruled by a self-proclaimed monarch, he joined in the conspiracy against his friend, which occurred on the ides of March. Antony eulogies his former opponent, "This was the noblest Roman of them all./ All the conspirators save only he/ Did that they did in envy of great Caesar;/ He, only in a general honest thought/ And common good to all, made one of them."