See the discussion on http://musicbrainz.org/edit/22518639. I encountered the same issue again when splitting the collaboration artist "Lord Finesse & DJ Mike Smooth".

ModBot used to enter such removals for the collabo relationships up until 2013-01-28, so the regression should have happened around then:

http://musicbrainz.org/search/edits?auto_edit_filter=&order=desc&negation=0&combinator=and&conditions.0.field=editor&conditions.0.operator=%3D&conditions.0.name=ModBot&conditions.0.args.0=4&conditions.1.field=type&conditions.1.operator=%3D&conditions.1.args=92&field=Please+choose+a+condition