Background apps launch overnight

I am noticing the same strange behaviour occurring once or twice per week with my 6s.

I have a bit of OCD about closing background apps, so I know for a fact that before I go to sleep I will not have any background aps running, yet once or twice a week when I wake up the following day, having checked facebook or the news etc, I go to close down the apps and lo and behold there must be about 15 apps running - and it is always the same ones (Strava, Maps, Photos, Calendar & Messages spring straight to mind, but there are others too. And they're always in the same order.

I cannot find any pattern with when it happens or what I might have done differently. It did it on Thursday night last week, and it did it last night (Monday night). It has been doing it for a number of weeks now, but I have only recently started to take notice of exactly when. I use the same (wall) charger every night, always around the same times.

Has anyone experienced this before or suggest why it might be? It's not a problem as such, just odd!

John