I think having them to be forcibly activated would be best, this way if you want to get a group together and try to do it you can pop right in and go for it, as opposed to possible having to wait 2-3 hours because you landed as one was finishing. As an aside to prevent people from farming it, after you complete it you cannot complete it again and get rewards for a week, so basically it would reset like the Raids and nightfalls do.