The Gemara has a question: Whats the chidush of that which we just learnt that dofen akumah works, there is a mishna later on that teaches us that. The Gemara goes on to quote the mishna by the case where the ceiling gets bashed in and then schach was placed over it.
The question is: Why did the Gemara ask specifically from that case and not from any of the other cases in the minsha over there???
Simcha, Jerusalem, Israel
It appears to me that the Gemara asked from the case of the ceiling that got bashed in because this is simply the first case mentioned in the Mishnah on 17a. Since this is the first of 3 examples there that are all based on the same principle of dofen akuma the Gemara decided to mention only this case.
Ketivah u'Chatimah Tovah