Have you checked your stirrup length? This can happen if your stirrups are too long sometimes. Otherwise it's simply a case of doing some stirrup less work to improve your core stability and to stop you from relying on your feet for balance.
Just focus on using your core to anchor yourself, and make sure to keep your leg on before the jump. If your leg isnt on in your approach, it is more likely to move over the jump. Bend at your hips, to throw your weight into your heels