Why is an arthropod especially vulnerable immediately after shedding its exoskeleton?

An arthropod is especially vulnerable immediately after shedding its exoskeleton because its new, replacement cuticle is initially soft and flexible.12

During this period, the animal lacks the protective, rigid armor that typically shields it from predators and environmental hazards. The new exoskeleton requires time to undergo a hardening process, which involves stretching and chemical tanning—a molecular stabilization of protein chains—to achieve its full strength. In some crustaceans, this process is further delayed as they must also deposit calcium carbonate into the new procuticle to regain rigidity. Because the animal is essentially "soft-shelled" while this hardening occurs, it remains in a highly susceptible state until the new structure is fully developed.12