Feedback happens when signal generated by the pickups (or microphone) is output from the speakers, then this is received again by the pickups and output again to the speakers, received again by the pickups and so on, creating a sound or feedback loop.
Chambered guitars create more harmonics and sustain which increases the amount of signal available to create this loop. Thus, archtop guitars are more prone to creating feedback than solid bodies.
I may stand to be harshly corrected on this, but that's my take on it. :) If I'm not mistaken, that is also why solid bodies were created in the first place. Obviously, they'd have tried putting pickup in acoustics first, then solved the feedback problem by coming up with the solid body.
...so ever since then, I always hang on to the buckle.
