It's usually compression that causes that.  If you are watching live video, realize that bandwidth plays a factor.  If your internet (either down or up speed) isn't optimal or you are using WiFi with a low signal, most video or streaming services will realize they don't have enough bandwidth to work with so as a result, they will increase compression to compensate and/or even reduce the resolution of the stream.  Having a good, reliable connection is key.

Mike