I've had runs of four or five successive disasters - flood, volcano, dust storm one turn after another. It's based on luck but modified by the land area you control. If you have one river, then there is X% chance of a flood. As far as I know you only have one disaster per turn. So if you have two rivers, then you have the same chance for each river, but the probability is now doubled.
But as bite said, it's the luck of the draw.