The order that PagePops appear to site visitors is based first on the start date, and then on the date of the most recent update.
The active PagePops will be shown in chronological order, with the most recent start date displayed first, and the oldest start date last. If two posts have the exact same start date, that's when the next criterion kicks in, and the post with the most recent update will be shown first.
Your best bet in controlling the order of your PagePops is by adjusting the start date of each one. Start dates can always be moved to a date in the past in order to force a particular PagePop to appear at the top of the list.