Board Thread:Questions and Answers/@comment-58.9.12.43-20130521141702/@comment-6034641-20130521181828

There are more conditions than having just an empty continent. For one, it tries to find a continent that wasn't used in the previous alert as well as has the most even territory control. If the server is very populated and even, it will also even attempt to find the most balanced continent in terms of population as well.

There is also a minimum window inbetween alert starting points to be sure that they arent happening all of the time. I believe that minimum window is 2 hours but I am not sure.