Besides for this being done very early, it is a major move. In the past four or five elections and Knessets, the Likud has signed this agreement with Yamina (in its various forms). Yamina signing now with Gideon Saar is a big move. 

Not only that, it also leaves Likud without any natural partner party to sign with. 

Likud could sign with Yisrael Beyteynu, as has been done a bit further back in election history, but Avigdor Lieberman and Bibi Netanyahu are not on such good terms nowadays, and neither are their parties.

Likud could theoretically sign with Zelikha's party, but polls so far don't have them getting into the Knesset, and they are more likely to merge with another party. I cannot see the Likud risk throwing out votes like that in what might be another close election in which every vote counts.

They could sign with Shas, as they are natural partners as well, but Shas always signs the agreement with UTJ, and I don't see that changing. It could if Netanyahu gets really desperate and Shas decides to save him, but it is not likely. 

The only other party that seems to be available to sign with Likud might be the party of Mansour Abbas, if he breaks Raam off from the rest of the joint Arab List. Netanyahu has been cozying up to Abbas recently and while it seems like a ridiculous idea, I can see it happening.