Why do non-caster DPS do this...?

[ForteNightshade]

Means people switch before the first target dies. Makes sense on caster since you could sit there casting and have your cast go to waste because the target dies before it goes off. But melee don't really have to deal with that yet still switch (perhaps to not clip their gcd but it's dumb if said target doesn't die). True thing.

Usually because they have a big hitting GCD that will do more damage than the mob has HP. So it's better to put that extra damage on something else instead of it going to waste. That, plus if it dies before their animation registers, it'll cancel.