Aha - found this after creating a new topic at: http://wordpress.org/support/topic/plugin-events-manager-events-manager-wipes-out-wp_title
I don't know what else Marcus has in the pipeline that might change this, but the fact is that having the EM filter on the wp_title hook interferes with the correct operation of the wp_title function.
wp_title is documented here: http://codex.wordpress.org/Function_Reference/wp_title and it takes two parameters related to a separator.
If I call:
wp_title( '|', true, 'right' )
then I should get back:
The filter in EM hooks to the wp_title hook, which is right at the end of the wp_title() function. The EM filter takes the wp_title() output and spits out some new output. HOWEVER...it ignores the separator parameters.
I would therefore argue that if you're hooking to the wp_title filter hook then your filter needs to observe the separator parameters to preserve the correct functionality of the wp_title() function.
The alternative is to hook the filter in elsewhere. I, personally, think that hooking into the_title and preserving the functionality of wp_title() is the right thing to do, but Marcus is the developer and he knows what's up his sleeve.