Apparently, boosters as event rewards do not go into effect until you read their inbox message. Event reward messages automatically pop up to be read one at a time, randomly, at event completion or login. So, if you have a stack of messages to be read, and you do not log into the game until you are ready to use the booster, you can delay when the booster goes into effect, thereby saving it for when you do have time.
If an event has more then one reward, getting all of them increases the chance of not triggering the booster until you are ready for it. If you do a bunch of invasions and infestations and their rewards are not yet in your inbox, you are adding to the pile of messages keeping your booster safe from premature activation.
So, before completing an event, do a bunch of invasions to get inbox messages queued up, that arrive after the event reward messages and sit on top of them. Then, complete the event for ALL the rewards so that hopefully something sits on top of that booster event reward and keeps it safe, such as the forma bp and badge for Shifting Sands tactical Alert Event. Then, don't log in until you are ready to use the booster. Each time you log in, you have a chance of that booster event reward message automatically opening, depending on the amount of unread messages in your messages queue.
I hope DE does not force the boosters to automatically activate at end of event. It would be a waste of boosters.