rabbit behavior

Had our 4 month old male rabbit outside in a run we made for him to exercise on warm sunny days. Suddenly he seemed very alert to something outside the run that I couldn't see and started thumping his back legs on the ground while he was in a sitting position. Is this a sign of aggression or warning or what?

My female rabbit does this when she's scared, maybe he could smell a cat nearby.

i have three rabbits that do this when they r frustrated or scared. to me its like a sound that might scare away unwanted visiters. its really the only thing they have.