A couple of key points here:
1) Resignations in the face of recalls are nothing new (just scroll to the post right below for one example). In 2012, 26 officials resigned rather than face a recall. There have been at least two state Senators who resigned in the face of recalls, Michigan's David Serotkin in 1983 and Wisconsin's Pam Galloway in 2012. However...
2) Colorado's law is very different than other states. It requires that the resigning official be replaced by a member of the same party. From a recall point of view, there are other state's that allow appointment rather than election for the replacement official, but Colorado may be the only one that mandates that the official serve in the same party.
3) Hudak's resignation shows the power of the gun rights forces on a few different fronts. The fact that she is resigning before signatures were handed in points out that she must have felt that there was an excellent chance that the petitioners met the standard (she would have had 5 days to resign after the signatures were handed in and still short-circuit the recall).
4) Hudak clearly thought that the voters would not have objected to flipping the Senate from Democratic control to Republican. If past recalls are any indication, she is right. We've seen it done in 1983, 1995 and 2012.
5) The sheer amount of signatures collected shows real strength. Petitioners needed about 18900 signatures. This is more than twice the amount needed for the Senate President Morse recall, and 5,000 more than required for the Angela Giron recall. Only one of the Wisconsin state Senate recalls required more than the 18000 (and Wisconsin's recall law is much looser).
6) In the past, Colorado recall petitions had the signatures struck down at a high rate. The Giron recall was a striking exception, due to the use of new technology. Are we seeing the same thing in place with Hudak -- and might this new technology make it easier and more cost-effective for initiatives and recalls to make the ballot?
7) As I mentioned before, these recalls were symbolic from the gun control forces point of view (that should not be thought of in the sense of meaningless -- many recalls are symbolic and it does not detract from their importance). Even if the recall has succeeded and the Republicans gained control of the Senate, there was little chance that the legislation would be enacted changing the gun laws. In that sense, a resignation may be even more powerful than a recall vote, especially since a Hudak victory would have at this point changed the narrative for the gun rights groups. Gun rights supporters can now claim to have effectively scared a Senator out of a seat after beating two others based solely on the gun control issue.
8) It should be noted that having a resignation occur the day before Thanksgiving is designed specifically to minimize coverage of the move. The signatures were due on December 3, and then there would have been further delays in checking them and possible lawsuits. It is possible that Republicans could have claimed that the replacement vote should not be short-circuited (it has never been tested before), but the law does read clearly on this front. Hudak had weeks to decide to resign. The day before Thanksgiving is chosen for a reason.