The twin jugs Carl threw after Katia's crowd blast are half the reason two soul crystals popped.
The opening line of Carl's reasoning is a reconstruction of what the jugs did. Katia Grim performed her Crowd Blast, he followed immediately with the twin boom jugs, and between them they killed several of the larger Krakaren Juvenile Cloneling at the same instant. Soon after, the two closest Soul Crystals popped.
He cannot separate the two explanations — simultaneous kills, or a lifetime load limit finally reached — but the combination is what he is gambling the endgame on, and the jugs are the part he can repeat.
Heavy improvised explosive jugs, thrown by hand.
Carl's heaviest thrown ordnance, and the trigger of the first soul-crystal detonation.
Also appears in books you've already read — spoiler-safe to their endings: