In the Mishnah, Rebbi and the Rabanan argue whether the unintentional killer is Chayav Galus in a case when his ax swing causes a piece of wood to fly off and kill someone. As the Gemara explains, the Rabanan maintain that he is not Chayav Galus, because they hold "Yesh Em la'Mikra," and thus they read the verse as " v'Nashal ha'Barzel Min ha'Etz" (Devarim 19:5), which denotes that the iron itself flew off of the handle and killed. Rebbi holds that he is Chayav, because Rebbi holds "Yesh Em la'Mesores," and thus he reads the verse as "v'Nishal," which denotes that the iron caused a piece of wood to fly off and kill.
The Gemara goes on to ask that we find elsewhere that Rebbi holds "Yesh Em la'Mikra ," and thus why does he expound the verse here according to "Yesh Em la'Mesores?"
We see from the Gemara that according to the opinion that maintains "Yesh Em la'Mikra," the word "v'Nashal" cannot be a transitive verb, expressing an action directed toward or performed on some other thing (e.g. the iron makes the wood fly off). This, however, is difficult, because we find that the simple form "v'Nashal" also is used as a transitive verb, in the verse, "v'Nashal Goyim Rabim mi'Panecha" (Devarim 7:1) -- Hash-m will thrust away the nations from the land. Thus, even if Rebbi holds "Yesh Em la'Mikra ," he can learn that the verse means that the iron caused a piece of wood to fly off and kill!
The Ritva raises your question, as well as the Rashash and other Acharonim.
1. The Ritva and Rashash answer that if the word, "v'Nashal," would be a transitive verb (according to the Rabanan), then the verse should say, "v'Nashal ha'Barzel Es ha'Etz ." The fact that the verse says instead, "v'Nashal ha'Barzel Min ha'Etz ," indicates that it is the "Barzel" that is falling from the wood, and not that the Barzel is propelling a piece of wood.
However, this answer seems problematic, because even according to Rebbi who holds "Yesh Em la'Mesores" and reads the word as "v'Nishal," the verse still says "v'Nishal ha'Barzel Min ha'Etz ," which implies that it is not the wood itself which broke off and killed the victim.
The answer to this is that if we read the word as "v'Nishal," then there is no other way to read the verse other than having the wood be the direct object of the transitive verb. Therefore, even though the words "Min ha'Etz" imply that the wood is not the direct object, nevertheless the word "v'Nishal" forces us to read the verse as if it said "Es ha'Etz." This is not the case, though, if the word is read as "v'Nashal," for then it is more fitting to interpret it as saying that the Barzel itself flew off, due to the implication of the words "Min ha'Etz."
2. We must note that the Yerushalmi (cited by the Ritva) learns that the Rabanan indeed learn the meaning of the word "v'Nashal" here from the word "v'Nashal" in the verse, "v'Nashal Go'yim Rabim." They explain that just as "v'Nashal" there means that the nations will fall due to the power of Hash-m's strike , so, too, "v'Nashal" here means that the Barzel falls due to the power of the person's strike of the ax; it is the person that causes the Barzel to fall from the wooden handle (and not the Barzel causing a splinter from the wood being cut to fly off).
According to the Yerushalmi, we can answer the question as follows. The Gemara understands that if Rebbi learns that "v'Nashal" here is a transitive verb from the word "v'Nashal" in the verse "v'Nashal Go'yim Rabim," then he should have explained that it is the striker that caused the Barzel to fly off from the handle, similar to the verse of "v'Nashal Go'yim Rabim," and not that the Barzel caused the wood to fly off. Thus, Rebbi learns that "v'Nashal" here cannot mean the same thing, even in the transitive sense, as the word "v'Nashal" in the other verse, and thus he must read it instead as "v'Nishal," with the Barzel being the subject of the sentence.
3. It is also important to note that the Rambam (in Perush ha'Mishnayos and in Mishneh Torah) has a different approach altogether to the Sugya here, according to which we can answer your question in a very simple way.
According to the Rambam, everyone agrees that the Barzel is what killed the person. The dispute between Rebbi and the Rabanan involves only whether it was the force of the descent of the ax (before it even hits the wood) that caused the Barzel to fly off, or whether it was the force of the blow on the wood being cut that caused the Barzel to fly off. Accordingly, everyone agrees that the words "Min ha'Etz" in the verse tell us that it was the Barzel that flew off and killed (as the words imply, as the Ritva wrote in the first answer above). When Rebbi says that we read the word as "v'Nishal," he is saying that the word "v'Nashal" implies that the Barzel flew off on its own without anything else exerting pressure on it (other than the mere speed of the descent of the ax) to remove it from the handle, while the word "v'Nishal" implies that some other force exerted pressure on the Barzel and caused it to come off the handle.
According to this explanation, we indeed do not find anywhere that the word "v'Nashal" has a meaning similar to "v'Nishal," and therefore Rebbi must base his Derashah on the Masores alone.