Often downloaded at the same time as an update to something you actually wanted - you need to be very careful in ticking/unticking boxes during updates.
Or you've clicked through to an interesting advert/site and again not noticed the 'install this useful toolbar' box.
Or they are entirely legitimate, but your AVG happens to have them on its list.
Or Mr Tilly has been on that "Big Bouncy Girls" site again ... [possibly]
Or it's cosmic rays, or the lizards ...
... basically 'it could be anything' - just let AVG get rid of them.