Just the enrage timer mostly. DPS needs to burn them down quickly or they enrage and one-shot everyone.
Also normal trash hits harder so make sure dps does normal -> silver -> gold or your healer will die.
That sounds a little weird having the weakest hit the hardest O.o Never would've thought of that doing them myself most likely though. While soloing I usually take out the normals first just because it gets damage off me quicker though.