When you paste a link into a mind map, MindMeister expands it automatically. If you're pasting a link from YouTube, the video is automatically embedded in the topic. If you're pasting a link to an image, the image will be embedded. If you're pasting a link to a website or blog, the featured image of the page will be embedded.
If you don't want links to be expanded automatically, you can turn the behavior off in your Preferences:
Click on your user avatar in the upper right corner of the screen and select Preferences.
In the new dialog that opens, remove the checkmark next to 'Automatically expand links'.