Have you updated wpForo to 1.3.0 and reset the Cache/CDN? This issue is already fixed with wpForo 1.3.0. Make sure you've updated, cleared and purged all caches and did Ctrl+F5 on front-end.
This is a conflict with your WP theme, if the issue is still not solved please put this CSS code in Forums > Settings > Styles > Custom CSS Code textarea. Save it, delete all caches and do Ctrl+F5 on front-end.
#wpforo-wrap .wpforo-post-content .wpf-video, #wpforo-wrap .wpforo-post-content .wpf-video iframe{max-width: 85%;}
@media screen and (max-width:1000px) {
#wpforo-wrap .wpforo-post-content .wpf-video, #wpforo-wrap .wpforo-post-content .wpf-video iframe{max-width: 98%;}
}