Is there a reason or explanation why the elements are all mixed up? They're all over the place.

Sivak (silver) explode in fire. So maybe there's an opposites theme? Nope, it's the only one that does an opposite damage type.

Baaz (brass) has a petrification gas explosion. Not quite the sleeping gas breath weapon of a brass dragon, but sort of in the vein of incapacitating.

Kapal (copper) explode with acid. A match!

Bozak (bronze) explode with force damage. Which I'll count as close enough to the bronze dragon's repulsion energy breath.

Autak (gold) has a lightning death throes, and a poison breath weapon. Neither match the fire breath of a gold dragon. However, the breath weapon can cause exhaustion, which is different from the gold dragon's weakening gas, but kinda in that direction.