I missed this when originally posted but foob4r's explanation was correct.
Baiscally I don't add any bittering hops until the boil has been underway for 15 minutes....the reason I do this has been lost in the sands of time but I think it may have been do do with the fact that when I boiled in a vessel that was pretty tight on headspace, adding the hops at the beginning of the boil generated a lot more foam and potential for boilover. Adding the hops after the initial break had subsided alleviated this issue.
Despite various equipment changes which now mean the issues is less likely, it has now simply become a habit for me....more a part of my process rather than any inherent recipe design.