I would say admins not keeping the forums content in order,
A few months ago I was looking at a supporters board for Premier League club, the main forum labelled Football Talk had a lot more topics about politics, the news and other stuff than football... The off topic section of the forum was almost empty.
I have a bit of the old OCD but if you were looking at that football forum and then look at this forum where topics are moved regularly - you appreciate how keeping the content in order not only makes it easier for members to find things but also potentially gets guests to register.
Obviously it's a bit different with the phpBB board as it's official, but for football boards where there are probably 4 or 5 the same all trying to attract new members organisation is important.
So after going on to look at a football match discussion I was side tracked in to joining the Monster Raving Loony Party instead.