You can train them out of it by using a pouch that is more open than a regular "pocket" style pouch. The theory is they are pouch protective as an instinctive thing, in the wild they would hear a noise outside their burrow and deem it as a threat, then crab in defense of their home.
In their cage, they can crab at anything: at you opening the cage, walking by, at another glider jumping on the outside of the pouch...basically any noise they hear but don't see can provoke the pouch crabbing. Some gliders get really into it and strike or lunge at the outside of the pouch as well. Personally, I don't consider it pouch protective if it's just crabbing..only if it's also accompanied by the cobra lunging in a glider that wouldn't do that outside the pouch.
If you use a parachute pouch (basically a square hammock) or a low profile bucket pouch, a tiered hammock, or any "open environment" pouch, something they can pop their heads up and see out of easily, it should reduce the behaviour. After a while, they'll learn there is no threat and once they settle down you can switch back to regular pouches.
Having said all that, though, i do think the term "pouch protective" has almost become a trend lately, with owners thinking any crabbing in the pouch needs to be treated. Some gliders are just crabby, some are just grumpy when woken or disturbed while in their pouch, and some just feel the need to be the cage alarm system. Only you can decide if the behaviour is dysfunctional enough to warrant trying a different pouch style to tame it.