Bertie who bounds

In a story by H. Munro a young man sings: Bertie, why do you bound?
As my Hornby explains, to bound means to move or run in jumping movements. What the lass does to make smb ask her that remains rather vague to me. Could you please clarify that for me?

If in the young lass was excited about something and had come running over, or something similar to that then the guy is just asking what she's so excited / happy about.