A couple of Elves were "betrayed" by one racist guy, basically he hurt their pride so they teamed up with the Naga, which is a bit of a mad move in any factions books.
Because of this one guy they changed their entire alegiance to the opposing side, including Orcs that have been killing them for years and trolls which they've been at war with for centuries.
I can understand Game-Wise why they swapped sides, becuase Horde REALLY needed a boost in faction numbers at the time that a "pretty" race would bring them (and it worked, so GJ Bliz), I can even see lorewise why they would leave the alliance.. BUT the lore reasoning to actually team with the horde was incredibly shallow at best.