If you include a URL in a contribution that matches an embedded site then it will be automatically updated to embed the content at the link. For example, a link to a YouTube video will result in the video itself appearing.
If you do not want this functionality then go to System settings and choose the Filtering option under Group content. Change the Automatic content embedding setting to No and this will no longer take place.
If you don't want to allow any external content in contributions then see How do I stop people using external media in content?