There are rare cases where a normal hyperlink to an external URL might get replaced to https. For instance, this could occur if the site’s own domain is part of the external domain. For example: if the external URL is http://domain.com.au, while the website URL is http://domain.com.
Really Simple Security replaces all instances of the own website domain in the HTML to https. In this edge case, the external URL gets replaced as well: replace http://domain.com in http://domain.com.au to https, and the external link is https as well.
This can be fixed by excluding this URL from the mixed content fixer: https://really-simple-ssl.com/knowledge-base/exclude-url-mixed-content-fixer/.
If this does not apply to your site, Really Simple Security won’t be the cause as it does not replace external URLs. In that case, we advise checking if another plugin might be causing this.