Actually I think that it has to do with how popular it is to hate the ultramarine's, in an attempt to be different many people copy others who also be different to the establishment or mom and dad figure (GW) and therefore rebel by hating that which represents it. A bit like Emo kids actually or anyone acting up to be 'Unique and individual' which in reality is just copying someone other than their parents or the established authority.
The short of it has to be that humans just aren't all that original.