Resigning is well, resigning. A person who resigns gives up his/her Mod position for different reasons.
I don't know the specifics for each ex-mod, but I can speak for at least 3 moderators who are leaving because Skyblock is quite simply, a waste of time compared to actual activities that impact your real life in a positive way, not in a negative way. Also, Skyblock gets extremely boring if you play for the Minecraft, not for the forming of lurking socialites.