ELFs and Sones have 2 of the biggest fandoms in kpop. I am both an elf and a sone, and I know what you're taking about. On EVERY snsd or super junior video, there is always a fight between the 2 fandoms. Its stupid.
There may be lots of reasons why they always fight.
1) They have no life, all they do is protect their favourite group, so if a elf sees a bad comment from a sone, they will go on a snsd video and hate on them.
2) Elfs think that snsd is hogging all the attention since snsd and super junior are from the same company. Snsd is more popular so their company focuses more on them and less on super junior, so they hate on snsd. (I guess its true, but theres nothing snsd or super junior can do about it)
3) Both groups are very close, since they are under the same company. They often have schedules together, and when you're with a group of people for a while, you most likely will become friends. Many fans are jealous of this, and hate on the other group saying things like "shes a whore, donghae is mine" or "ew shes not even pretty, why is he friends with her?"