On all channels it keeps going back on its own 30 seconds or more. Sometimes it goes back to the beginning of the program. This is very annoying!
These things are just little special purpose computers. Like their big brothers sometimes clearing things out by restarting them is all it needs.
Restart via Roku menu path Settings > System > System restart (Or for Roku TV: Settings > System > Power > System restart - just turning the tv off/on doesn't give a full restart)
If a restart doesn't do it, or if you don't have sufficient control with the remote to navigate the Roku menus, you can also restart by pulling the power plug to the device for a minute or so then plugging it back in.