Those people directly involved in this discussion would had been in the best position to give you a direct answer because all we'll be doing here is speculating on what the reasons are without actually knowing the main reason. People can move from one campaign to another campaign for various reasons. Since you have already ruled out the reason of better pay rate lets move on to the managers involved with those campaigns and also the weekdays of the campaign. Some people prefer campaigns that ends during the weekends while others love campaign that ends during the weekdays. As for the managers, people have their favorites and they'll always love to work with them because of their fast payment and on time receiving of payment without having to wait hours before the coins gets to your wallet due to poor network fees or other managerial behaviors.
This particular situation has higher pay reason all over it and there's nothing wrong in that, there's a new campaign that'll pay you more for your contributions on the forum, it's only normal for him to want to partake in such campaign. Getting paid more for the value you bring has a better feeling so there's nothing wrong in anymore moving to experience that feeling. Being in a campaign for so long could be as a result of the piece of mind that the campaign gives you, knowing it's a long lasting campaign and you won't have to bother about applying for a new campaign often. Looking at the Campaigns that have escrow addresses you can easily predict how long the campaign would last and determine if it's worth moving to the campaign or not.