I've just found out from a reliable source that Cortese's nickname at school was 'boo'...something to do with him having a girl's name and the other lads trying to make out he was a bit of a girl's blouse when they shouted 'boo!' at him. Whilst initially derogatory, it stuck as a nckname and evolved into a term of endearment over the years.
So, it turns out there were simply a few "in the knows" in the crowd who shouted "boo" when he appeared on the screen as a mark of respect.
At least we'll all know to join in next time.
Thread can be closed now we've cleared that one up.