For me, it started when I noticed a trend between United and Liverpool fans bleating about 'not being supported in Europe' by other English clubs, yet when other English teams competed in European competitions (CL and UEFA Cup) they would mock their failures mercilessly.
A case of "do as I say, not as I do". If they weren't going to support other English clubs, why should rival fans do the same? And yeah, they tried the whole "ABU" bull, but the fact was they couldn't hide their hypocrisy. I noticed other clubs fans' all doing the same to one another soon after that.