I had a similar issue and I had to dig into my HTML code to see what was happening.
The Theme for my Moodle site (2.9) doesn't always make hyperlinks look pretty, so I manually added Underlines for emphasis. In my case, I had created a Page resource added some text, and linked 4 .mp4 videos from our back-end server. I set them all to open in a new tab/window.
Parts 1,2, & 4 linked just fine, but Part 3 embedded itself in the Page and autoplayed.
I eventually looked at the HTML to see what was happening.
In 3 of the 4 instances, my tagging went something like this: <a href=...><u>Part #</u></a>
Part 3, somehow, went like this: <u><a href=...>Part 3</a></u>.
As soon as I re-ordered the tags, everything worked the way I wanted it to. No idea why it should make a difference but it did.