Good question.. I'd say its cause people wanna jump on bandwagons and follow the leader... Everyone was bumpin Jarule back in the day before 50.. then when 50 starts dissin ja, all those people that were playing "Down Ass Chick" and "Holla Holla" all of a sudden aint feeling him.. since 50 was the up and coming next big thing, people just didnt want to be left behind rooting for the "old news"...Which leads to me to believe that all those Ja haters that did like him at one point still bump his shit once inawhile at home, buy/download his new songs, but will never admit it since they might think they'll face ridicule..
But like you said, it doesnt just pertain to Ja/50, or anything music related.. I think most people hate just to fit in or be cool with the enemy or rival of the underdog.. That way it'll increase their chances of ever facing a confrontation..