Feedback happens when the sound from the amp causes part of your guitar to vibrate. The pickups produce a signal for the amp when the strings move relative to the pickups. Whether that movement is due to your picking, or due to the sound from your speaker doesn't matter.

You can try putting some soft foam under the pups to keep them from flapping in the breeze, but you may simply have to be more careful about how you position the guitar relative to the speaker.

You might also roll back the gain a bit, but that's not as much fun. Feedback can be a useful effect, if you can control it. Infinite sustain! Try working with it.