Let's get one thing out of the way first. We have no evidence that hoplites engaged in mass shoving. There's no literary work that says this and no vase or statue that shows it. The mass shove is a strictly theoretical model of hoplite combat, which was first launched by G.B. Grundy in 1911 and has been controversial since 1942. There's a tremendous pile of scholarly literature on the subject, which I won't bore you with. The main thing is this: the easiest answer to your question is that such shoving never happened, so hoplites never needed special protection. We don't know if this is the right answer. But it is as legitimate an answer as any other.

Now, let's make things more complicated.

Why do some scholars argue that phalanxes engaged in mass shoving? Mainly it's because a few surviving battle descriptions involve variants of the verb "to push" or the noun "the pushing" (othismos). These words unfortunately don't explain themselves, and there are different ways they can be interpreted. Grundy and his followers took the word to mean that the entire hoplite formation got together and pushed its way forward, bowling over and trampling anyone who could not muster the cohesion and strength to shove back. There's some arguable supporting evidence in the use of words like the "weight" of formations and in the description of the fighting style of later pike phalanxes, but I'll leave all that aside for now.

In this model of hoplite combat, the hoplite's armour and weapons actually become an encumbrance; all he's supposed to be doing is push the man in front of him with all his strength. The force exerted on the man in the front rank - shoved by 5-50 men from the rear, and with as many men pushing into his front - would be absolutely crushing, and there would be no room for him to wield a spear or a sword. The man's only purpose is to stay upright and keep directing the force forward until the enemy gives way.

Any sensible reader would see this as a death sentence for the front rank, and so a considerable amount of scholarship is devoted to the question how such a mass shoving match would be feasible. The latest research uses comparative evidence from Japanese student riots (J.E. Lendon), Copenhagen riot police (A. Schwartz) and biological swarm behaviour (P. Bardunias) to show that in certain situations, masses of living creatures can work together in a way that amplifies their momentum rather than turning its members into squishy goo. There are physics involved in these arguments that I won't pretend to understand, but the point is that each of these authors reasserts the view that a mass hoplite shove is survivable and even effective as a combat tactic.

The key element that such arguments rest on is the hoplite shield. It's often been noted that this shield, with its peculiar shallow bowl shape, is both unusual in world history and strikingly uniform across the Greek world. It's also been noted that its design is pretty ineffective, in terms of how much weight is concentrated in the rim without contributing much to the shield's protective area. Those who believe that all things must have a practical explanation have argued that these aspects of the shield only make sense if it served the purpose of helping the hoplite survive othismos. When held in front of the body, the bowl shape covers all of the torso, but the offset rim pushes that cover somewhat away from the body. This would allow the hoplite to retain space to breathe even when subjected to great pressure from the front and rear. They would therefore remain alive and perhaps even able to use their spears as the rear ranks laboured to push the enemy from the field.

At this point I should stress again that the mass shove is a purely theoretical notion that is not confirmed by any source. All this scholarly ingenuity may be wasted; all these arguments were devised to support an interpretation that may or may not be the right one. They may deflect the criticism (already raised by A.D. Fraser in 1942, and many times since) that a mass shove would crush hoplites to a pulp rather than allowing them to fight their enemies, but they do not prove that such mass shoves actually happened.

The alternative interpretation, championed by scholars like P. Krentz, A. Goldsworthy and H. van Wees, is that the references to pushing in Greek battle descriptions are either individual or metaphorical. In some cases they clearly refer to individual warriors using their shields to push or knock enemies off balance; the rear ranks aren't involved. The great majority of depictions of hoplites in combat also show this kind of pushing, with the hoplite holding the rim of his shield away from his body for greater protection. There is no need for special equipment in this case, since there is no pressure from the rear - only from the man in the front rank of the enemy phalanx. In other cases, since the "pushing" isn't explained in any detail, it's possible to read it simply as a metaphor, in the same way that an army may "make a push" nowadays - that is, they advanced en masse and the enemy gave way. In such cases, again, there is no actual shoving from the rear, and in any case the moment of contact is usually very short. This reading has the great advantage that we don't need to check with the Copenhagen riot police to see if it might be physically possible.

Obviously there's still a lot of works out there that take the notion of a mass shove for granted. If you find that model believable, the answer you seek is that not all crowd dynamics lead to death by asphyxiation, and the hoplite shield appears to have been uniquely suitable for a mass shove (although Assyrian heavy spearmen used a similar shield, and no one ever argues that they engaged in mass shoving). However, the matter is not settled among scholars, and likely never will be. The simpler explanation is that we don't need to explain how a hoplite could have survived a mass shove.