In a situation where 2 groups of witnesses contradict one another about the event, it is classified as עדות מוכחשת where we have no reason to believe one any more than the other. Under these circumstances the gemara in baba basra has a discussion about what to do - it is a safeik so follow chazaka, but one thing is clear, we don't believe the later group any more than the first. However, if the second group doesn't testify about the event, rather about the validity of the first 2 as being 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 chiddush, since everything that the first group is saying is true, just that by believing the second group that the first are thieves, we automatically don't accept their testimony.
Rava (in the first lashon) holds that eid zomeim is a chiddush and therefore only becomes passul from the time of the hazama, and not retroactively from the time of the testimony. Abaye would presumably agree with rava that eid zomeim is a chiddush, just that it makes no sense to passul them from the time of the hazama, therefore we passul them retroactively from the time of their testimony. It seems that the concept of "chiddush" by eid zomeim, is that rather than considering it to be a case of עדות מוכחשת 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 Tosafos who explains that the chiddush of eidim zomimin, more that contradictory testimony is either that the second group is entirely believed, or that the first group is definitely passul not just a safeik. I am assuming like tosafos second approach that the chiddush of eidim zomimin is to view the testimony to be on the character of the witnesses, not on the event, in which case it is not a chiddush to directly passul the first or be machshir the second, rather it is a chiddush in classification).
Why is eidim zomimin somewhere in between? In essence the second group are not making a character judgement, they are only contradicting the facts - "these 2 could not have possibly witnessed what they claim to have witnesses since they were with us elsewhere". Had it not been for the chiddush Hatorah that we believe the second group, we would view it as if they just contradict the first group about the events, where we have a real safeik who to believe. We would interpret their intent to simply be "the event was not witnesses by these 2 eidim because they were with us elsewhere". But the torah is mechadesh that we are not to regard the hazama 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? Because when testifying about an event it is sometimes possible to misinterpret the events, or not have a clear picture as to what actually happened, so we give each group the benefit of the doubt. But, by eidim zomimin 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 gazlanim do, and therefore they are not admissible as witnesses in any court.