Rabbits have long, powerful hind legs  with well-developed muscles that provide the necessary force, acceleration and maneuverability required in case they sense dangers such as seeing a predator.
They also enable them to hop, make a long-jump or swiftly escape. Also, bunnies may use these hind limbs to try to break free if they are held especially if they are so afraid or they do not want to be held.
If you watch your bunny carefully, you will sometimes notice him stand upright only with his or her hind legs. What message is your furry friend trying to pass to you or other rabbits?
Why do bunnies stand on their rear legs?
This is normal behavior or rabbit’s body language and if you notice it, your pet may be trying to check or look around. Standing up this way makes her or him have a better view. He or she could be looking around out of curiosity, to see if there is food or if the place is safe.
Besides that, “indoor pet rabbits may also stand up when they want to see items placed up high in the house, ask for a door to be opened, or beg for food.”
Therefore, when a rabbit is standing on its hind legs, it could be trying to tell things such as ‘I am looking around to see if it is safe’, ‘I am checking to see if there is any predator nearby‘, ‘I need you to open the door’, or ‘I need food’ and so on.
Finally, this might be only an effort to try to reach something that is high since when they stand with their hind legs, they become taller.
However, due to the slight variation on how these pets communicate, this behavior could also mean that he is properly curious or wants to have a better view and see what is going on.
Loud noise including those of predators may often make this furry creature want to look around and to do so, he or she might stand on her rear legs.
However, some conditions such as sore hocks, trauma, hind limb paralysis, and musculoskeletal disease including arthritis can often impede this body language.