Comment by dugidugout

I believe it was emergent from FPS gaming culture, particularly following the popularity of Apex Legends. In Apex Legends you have an energy shield which serves as a buffer of hit points. When playing cooperatively it is useful to communicate when this energy shield is "cracked", thus the line "they are cracked" emerged. This originally meant a target player's shield is down in Apex Legend specifically, but it was then the Fortnite (and broader FPS) community which took this phrase and warped it to mean someone is precise or an excellent shot. Today it is certainly used in the context the original poster intended.

edit: Looking again, this may be overstated. Apex-era gaming culture likely helped popularize the usage, but considering older idioms like "crack shot," the actual etymological root is more likely there.