Why does the Apple Watch health reminders pop up at 50 minutes past the hour?
The Apple Watch wants you to stand each hour to complete your stand goal. So if you haven't stood during all the hour, it will remind you to do it 10 minutes before the next hour to give the the time to stand a little and add an hour to your stand goal !

It's anyway what apple says on their website : 


  Even if you’re active part of the day, sitting too long has its own health risks. So Apple Watch senses when you stand and move just a bit and gives you credit. And if you’ve been sitting for almost an hour, it reminds you to get up. You close the Stand ring when you’ve stood and moved for at least one minute in 12 different hours during the day. It may sound like a small thing, but sitting less can improve your health in significant ways.