Higher pay is of course the main reason for members who switch to signing campaigns, besides project uncertainty, convenience with the rules set by the manager are some of the other reasons for members switching to signing campaigns.
i pay enough attention to members who often move, there are members who have not received a single campaign until now for several reasons and maybe the manager marked their account and put their account on the blacklist.  Apart from that, i'm sure you've got the answer to your question.