Small Animal Channel explains that crabbing indicates fear and serves as a defense mechanism in sugar gliders. Fear-related behaviors can be minimized by removing stressful stimuli from the environment. If a sugar glider crabs when he sees the pet cat, keep the glider's cage where the cat cannot reach it.
In addition to crabbing, sugar gliders use a variety of body language cues to communicate stress. A stressed or fearful sugar glider may stand upright on its hind legs and open its mouth to appear larger and less like prey. Chronically stressed sugar gliders are often irritable, lose their appetite, self-mutilate and dislike handling, so it is important to keep gliders in a low-stress environment for their health.