Thanks csw14, well done! Coincidentally a pro user figured it out last night.
The reason for the 100 priority is to make it work with Yoasts SEO plugin.
I still maintain this is a StudioPress' fault. It shouldn't matter where my filter is added, at worst my changes would be ignored but not cause the bug above. Am guessing they generate the title tags within the title during wp_title, which is wrong. I've emailed them about it, if a member wants to invite them here to figure it out, feel free!
However, here's an easy fix for those with studiopress without hacking:
remove_filter ( 'wp_title', 'em_content_wp_title',100,3 ); add_filter ( 'wp_title', 'em_content_wp_title',10,3 );
insert that e.g. in your functions.php file, or create a new php file in wp-content/mu-plugins/ and add it in there (between php tags of course)
that said, once events become posts, it'll be fixed.