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".