Shakespere chooses Brutas to be the last to stab Caesar and they had been friends. So while Caesar wasn't exactly shocked that all of the others wanted him dead - he was suprised about Brutus, exclaiming Et tu, Brute - meaning 'you too, Brutus'.
Casca is the first to strike, and, after each of the conspirators attack Caesar, Brutus is the last to stab him. The script of the play does not indicate any other order, except that all of the conspirators do stab him at least once.
