That's just a YouTube buffering issue, and is fairly common.
When the web page is loaded, your browser has not downloaded the whole of the video, just the first few seconds.
As you start watching the video, the rest of it starts downloading in the background. A red bar indicates your progress in watching the video, while a gray bar indicates how much of the video has been downloaded. If the red bar ever catches up to the gray bar, YouTube has to pause and wait for more of the video to become available. That's why you never want to see the red bar catch the gray bar. (Immortalized in the comic below.)
Simply put, your ISP can't transmit the video as fast as you can watch it. (I hear that some ISPs might purposefully throttle YouTube traffic.)
As to how to keep it from happening, pausing the video and waiting for the rest of it to load before resuming is the simplest way. For a more robust solution, there are several questions in the youtube tag here that ask about it with some possible solutions (and to which this one is probably a duplicate). Super User also has a bunch of questions tagged