[Archer has seen it too, for while he had his hands full of barely conscious Caster at that time, he wanted to make sure the sigils are destroyed.]

It's impossible to tell without knowing more about this spell. It could be because those sigils were the last left or absorbed most energy. It could be what was supposed to happen if the amount of energy was high enough. [Archer shakes his head and frowns.] Aside the fact that the spell drained all the victims we know nothing of its purpose. It could feed power into its intended recipient, or use it to fuel another spell.