People using the HTTPS Everywhere extension will have the site URL as https, but not the embedded videos.

since the page is also available via https, using protocol relative urls for the embeds should fix the issue (src="//www.youtube.com/embed/SOpwHaQnRSY" instead of src="http://www.youtube.com/embed/SOpwHaQnRSY")

As mentioned in another comment, we recommend using schema-less embeds, e.g. "//www.youtube.com/embed/...". Regardless, these embeds should still work when embedded over HTTP.