Rabbits have really good hearing. There was something outside spooking Shandi out but I couldn't hear anything. When rabbits get scared, they stomp to alert other rabbits of possible threats.

To calm my house bunny down, I usually do the following things:

let her hide in her box until the noise stops
sit with her and talk to her, reassuring her everything is ok
distract her so she forgets about the noise
offer her some food

If that doesn't work, putting some music on to drown out the noise helps too.