Eros, mentioning of Kakashi's in Anbu brings a thought that I've never had or heard and think it "might" be a possible reason for Kakashi's MS.

Ok, Kakashi was in Anbu at the same time as Itach was...He's only 9 years older than Itachi... Now i'm thinking this, that while they were both in ANBU, Itachi was 13 and captain, Kakashi was 22, that's 9 years after Obito's death...

What if Kakashi and Itachi were very close during this time, and perhaps trained with the Sharingan together... Itachi may have taught him a few things on how to use the sharingan... Itachi's crime and leaving ANBU and Konoha might have been a shock to Kakashi and perhaps his last bond broken... This could be a reason why Kakashi took interest in Sasuke because he reminded him so much of Itachi (when Sasuke and Naruto were revealed as Itachi's student - which seemed planned - we can understand Minato was the reason for Naruto, but we didn't know why the interest in Sasuke)

Now, Itachi comes back to the village after so many years and has the fight with Kakashi, and causes Kakashi to be bedridden for weeks... this might be even more hurt to kakashi than anything... If this is the case, then it could awaken Kakashi's MS... (that is if kishi has been hiding their relationship this entire time and would reveal it in a flashback)

Oh well, just thought it's an idea that's worth putting up although it may not be that strong...