Praying mantis, and by extension all arthropods, molt to grow. The exoskeleton, while strong and protective, is rigid and inflexible. This inhibits your mantis from growing larger than the exoskeleton allows. In order to grow, they need to shed and slowly form a larger one. 
Injured mantids even with missing limbs can regenerate the limb or previous mismolt!

We do find many mantids will molt after being shipped.

Seemingly the changing conditions can trigger the molt. We are studying this more closely as to determine the conditions which may induce the molt. Debatable whether it may be the stress, temperature changes or one theory I have is with Air-Delivery, as the barometric pressure changes would have an effect. Compressing and expanding especially in cargo would be extreme to the mantis. Most of us know the effects on ones ears even in a carefully pressurized cabin during flight.

Molting in insects is, by definition, the process of shedding and removing the exoskeleton. The exoskeleton is an external skeleton found in arthropods; where vertebrates like mammals and birds have an internal skeleton, insects, crustaceans and arachnids have a solid, outer skeleton known as the exoskeleton.

Why Do Mantis Need To Molt?

Praying mantis, and by extension all arthropods, molt to grow. The exoskeleton, while strong and protective, is rigid and inflexible. This inhibits your mantis from growing larger than the exoskeleton allows. Animals with exoskeletons are essentially trapped in their own skin, so in order to grow, they need to break out and slowly form a larger one. 

A secondary reason for molting which you see in longer-lived animals like tarantulas, is to renew their damaged exoskeleton. In other words, some athropods molt to heal injuries. 

Madagascan marbled mantis nymph (Polyspilota aeruginosa).

His previous exoskeleton.

The Signs a Mantis Is Going To Molt.

There are several indicative signs of an approaching moult in mantises. Refusal of food, lethargy, erratic vibration-like movements, inflation of the abdomen, elongated and extended legs, open raptorial arms, and large, swollen wing buds in sub-adult mantids.

It is perfectly normal to not eat for days when mantis being in pre-moult. This lasts just a few days

The most telltale sign of an approaching molt is in the inflation of the abdomen, especially if they have also been refusing food. Praying mantis absorb a large amount of air in their abdomen before finally molting. 

Additionally, mantis in pre-molt will hang from the top of their enclosure, however this is typical behaviour that can be difficult to differentiate from how they normally behave. Other signs worth noting include slightly parted raptorial front arms and elongated, extended body & legs as they stretch down from the top of the enclosure. 

You should almost never disturb in the molting process. After your mantis sheds , it will hang, suspended in the air, attached only to the previous skin by its lower abdomen. While it may look stuck, this hanging phase is called hardening.

Hardening is the process of “fortifying” the new exoskeleton after removing the old one. Internal secretions are released which harden this new exoskeleton into a defensive exterior like the last. Between the molting and hardening phase, your mantis undergoes a short burst of growth while the new exoskeleton is soft enough to allow it. 
We welcome your input with this information and  have heard from keepers and entomologists who contribute 

The mantis is now extremely soft, vulnerable, and should not be handled or fed for about 48 hours. 

Read more on molting here on our blog