Can I please ask a question? So, around the 12/13 second mark, why did they do that?

they rolled over so he could do the other side lol

I'd think it has to do with people using a lifeguard signal as some sort of generic time interval switch. e.g. they expect the lifeguards to be warning people every 10-15 minutes so when they hear the whistle they roll over so they don't over tan/burn themselves on one side. sorta like the people have programmed themselves to do the flip and they don't really fully wake up for it. the timing doesn't really matter to them so long as it's fairly regular and consistent. that's a best guess and it can easily fall apart under scrutiny, but I think there's a shred of sense to it.