In a situation where two groups of witnesses contradict one another about an event; it is classified as contradictory witnesses, where we have no reason to believe one any more than the other. Under these circumstances the Gemora in Bava Basra has a discussion about what to do - it is an uncertainty, so follow the chazakah. One thing, however, is clear, that we do not believe the latter group any more than the first. However, where the second group doesn't testify about the event, rather about the validity of the first two as being valid witnesses, such as testifying that they are thieves, the second group is completely believed to overthrow the testimony of the first group. This is not considered a novelty, since everything that the first group is saying is true, just that by believing the second group that the first are thieves, we automatically do not accept their testimony.
Rava (in the first version) holds that a zomeim is a novelty and therefore only becomes disqualified from the time of the hazamah, and not retroactively from the time of the testimony. Abaye would presumably agree with Rava that zomemin is a novelty, just that it is not logical for them to be disqualified from the time of the hazamah; therefore we disqualify them retroactively from the time of their testimony.
It seems that the concept of “novelty” by zomemin is that rather than considering it to be a case of contradictory witnesses, where the second group are merely disagreeing about the event, we consider it as if the second group are actually testifying about the character of the first group, invalidating them as witnesses. (See Tosfos who explains that the novelty of zomemin more than contradictory testimony is either that the second group is entirely believed, or that the first group is definitely disqualified, not just out of uncertainty. Assuming like Tosfos’ second approach that the novelty of zomemin is to view the testimony to be on the character of the witnesses, not on the event, in which case it is not a novelty to directly disqualify the first or to validate the second, rather it is a novelty in classification).
Why are zomemin somewhere in between? In essence, the second group is not making a character judgment; they are only contradicting the facts – “these two witnesses could not have possibly witnessed what they claim to have witnessed since they were with us elsewhere.” Had it not been for the novelty of the Torah that we believe the second group, we would view it as if they just contradicting the first group about the events, where we would have a legitimate doubt as to who to believe. We would interpret their intent as simply being that the event was not witnesses by these two witneses because they were with us elsewhere. But the Torah teaches us that we are not to regard the hazamah as just undermining the plausibility of the event, rather they are giving a character testimony similar to claiming that the first group were thieves. Why?
It would seem that the reason is because when testifying about an event, it is sometimes possible to misinterpret the event, or not have a clear picture as to what actually happened, so we give each group the benefit of the doubt. But, by zomemin, the second group is claiming that it was clearly premeditated lying that is taking place, not an innocent mistake. People who would fabricate a story when they were in an entirely different location have a fatal character flaw just as thieves do, and therefore they are not admissible as witnesses in any court.