Ok. this is a problem I noticed today. Let me explain the situation,
I have embedded a YouTube video on one of my client websites. That video is placed at the end of the page. And when the page is loading the page jumps to that video instead of loading the page from the top. This is happening without me entering any anchor tags in the iframe of the YouTube embed code.
When checking the iframe code I noticed it is loading a # tag called #document within the iframe. This is causing the page to jump.
Add following code to prevent #tag in iframe from loading on page load,
style="display: none;" onload="this.style.display='block';"
If your using WordPress this method is more suitable, Add the following code in the footer section. This will make the page jump back to top.