Well, I'm no mouthpiece genius. But I've played my share of mps with tips smaler then I'm used to.
And I think the way it works is that, since the reed your using is too soft it vibrates much more, and when you push the reed harder the reed vibrates harder and eventually will touch the reed tip and close up not allowing any more air through the mouthpiece. Then once you let up on the reed a little bit I'm guessing that all the air trying to come out of your mouth all comes out at once when the reed lets up. And since a lot of air is trying to go through that small opening it causes the reed to vibrate a lot faster. Causing the squeak.
I'm not sure if thats correct, but I think that what happens.
And I think the way it works is that, since the reed your using is too soft it vibrates much more, and when you push the reed harder the reed vibrates harder and eventually will touch the reed tip and close up not allowing any more air through the mouthpiece. Then once you let up on the reed a little bit I'm guessing that all the air trying to come out of your mouth all comes out at once when the reed lets up. And since a lot of air is trying to go through that small opening it causes the reed to vibrate a lot faster. Causing the squeak.
I'm not sure if thats correct, but I think that what happens.