I would like to answer the question I asked on Tophot a few days ago in Bava Kama page 29b. The reason Tosphot assumes the beginning of the braita is talking about a case in which the fall was not his fault is that that is the assumption at this point in the argument. [(That braita says if his jar broken and he did not pick it up, R Meir said he is liable and R Judah said no)] R Yochanan said they disagree after the time of the fall. At this point the Gemara suggests that during the time of the fall both agree that he is not liable. That is why Tosphot holds at this point in the argument that the Gemara assumes he fall was not because of his fault. Then the Tosphot asks that if in fact that beginning of the braita would be talking about a case of pure accident, then there would be a contradiction in R Meir. For here he would be saying after a fall by accident he is liable, while in the end of this same braita he says that after a fall by accident (and his vessels caused damage) that he would not be liable. So, in the next part of the Gemara where the Gemara suggests that during the fall both agree he is liable, at that point we would assume the beginning of the braita is talking about a case of a fall because of negligence and that there R Judah would agree he is liable