If only to bring enlightenment to this mystery, I would like to add the following explanation from an objective perspective:

(1) Decomposition in dead animals can be hastened by warmer temperature but the shipment occurred during winter (December 27, 2019) on a 20-hour air shipment (less than a day) when it was cold.

(2) Also, severe melanization doesn’t happen in live shrimp in 20-hour. It takes time for melanization to develop and become severe. More so when caused by blunt trauma leading to secondary infection on the anatomical parts of the shrimp prone to impact injury.

(3) Even those pathogens that are known to stimulate a direct melanization-defense reaction in shrimp take time to develop e.g. Fusarium and Paramoeba.

(a) Fungus
https://www.researchgate.net/figure/Images-of-L-vannamei-gills-A-Normal-gill-of-L-vannamei-B-Black-gill-of-diseased-L_fig1_283655004

(b) Protozoa
https://www.aquaculturealliance.org/advocate/detection-of-an-amoebic-parasite-in-cultured-paci%EF%AC%81c-white-shrimp/

(4) And the lesions caused by these ubiquitous opportunistic Fusarium and Paramoeba occur on the parts of the shrimp not prone to impact injury i.e. gills inside the gill chamber that is covered by the carapace.

(5) Finally, Localized chronic septic necrobiotic melanization as a healing reaction of the injured shrimp due to blunt trauma injury takes time to develop and this condition is not a contagious condition and the other shrimp in the bucket are apparently healthy lacking any manifestation of blunt trauma injury.