Malign fate is usually cited as the cause of their deaths, as it is a string of unfortunate coincides that lead to it (the message from the friar going astray, Romeo's friend seeing Juliet "dead" and rushing striaght off to tell him). Of course these are communication problems (if only the message had been sent sooner, if only his friend had spoke to the friar before hurrying off to Mantua...) but Shakespeare was writing a morality tale about the danger of suppressed love, and of course his greatest love story, and to simply have the characters commit suicide through something as simple as a "lack of communication" would have left the audience unhappy. That they are fated to die is apparent at the beginning - they are "star cross'd" lovers, whose deaths will end the period of civil strife caused by the animosity between their families.