This is because SmartRhythm operates as a "Motion and Fitness" activity to be able to collect activity and heart rate data. It is a limitation of Apple Watch OS that doing so forces our app to be placed on top. We've tried to make this as unobtrusive as possible by adding date and time to the Kardia watch face.
You can turn off SmartRhythm monitoring at any time to not have it take over the home screen. Learn how here.
Do note that turning off SmartRhythm turns off the heart rate collection from the Apple watch sensors, and also will turn off the ability to create notifications based on your heart rate and activity patterns.
With SmartRhythm turned off, you will still be able to record EKGs on the watch.