Feedback is caused when amplified sound leaks back out of the ear, is captured by the hand and re-amplified.Hearing aids can’t be totally air tight as that would be uncomfortable and result if poor sound to the wearer. To get an idea of what I mean, plug your ears and speak. Your own voice will echo. Venting helps reduce that, but makes a path for sound to leak back out. Today’s hearing aids are better at reducing feedback through digital cancellation, but it can still occur. There is also a trend to more “open” fittings that mean more leakage. In general, the worse the loss, the more likely you are to have the feedback.