In coming days, I may have more substantive thoughts on the issues raised by the likely (though still by no means certain) overruling of Roe v. Wade. Here, however, some thoughts on the possible identity and motives of the leaker. I should warn, however, that this speculation could be wrong, and I could be missing some potential scenarios. Indeed, these thoughts may not be worth much more than you're (not) paying for them!
Herewith some scenarios, in rough order of my estimate of their likelihood.
The leaker is a clerk for a liberal justice. The motive is fairly obvious: The clerk hopes that the leak will stimulate a vast public outcry that could cause at least one of the five justices supporting Alito's opinion to reconsider; perhaps because of concern for the Court's public legitimacy, his or her own image, or both. The obvious problem with this theory is that the conservative justices know perfectly well that overruling Roe will make many liberals/abortion rights advocates mad. The strategy could even backfire by making them more determined to stay the course, so as to show they won't yield to pressure. However, the leaker could differ with my assessment on that point. He or she might even have inside information on what a key swing justice is thinking.
A clerk for a conservative justice who wants to overrule Roe. The motive is less obvious than in 1 above. But I think this scenario is only slightly less likely. On this theory, at least one of the five conservative justices backing Alito's opinion is wavering (perhaps Kavanaugh or Barrett?). The clerk wants to raise conservative expectations of victory, and signal to the "weak link" that conservatives will view them as a traitor if they vote the wrong way. Worse, the justice in question will be seen as having given in because of intimidation caused by the leak. The latter may occur because people will assume scenario 1 (described above) happened. If this is the plan, I doubt it will succeed. But the clerk in question may calculate things differently from me or even have relevant inside information suggesting the plan could work.
One of the five conservative justices backing Alito is the leaker. The motive is the same as in 2. I think this is improbable because justices are less likely to leak than clerks, for any number of reasons, including that they must coexist with the other justices for many years to come. But not completely impossible. I rank this ahead of 4, in part because there are simply more conservative justices than liberal ones.
The leaker is a liberal justice. Motive is the same as in 1. It's worth noting that Justice Breyer is about to retire. So he may not care if his colleagues get mad at him for leaking (he's almost out the door anyway!). That said, such a step would be highly out of character for Breyer (unless what we think we know about his character is completely wrong).
Chief Justice Roberts or one of his clerks is the leaker. The motive is to pressure one of the five conservatives into backing Roberts' strategy of upholding the Mississippi law without completely overruling Roe. I think this is highly improbable, in part because it would be out of character for the cautious Roberts, and also because Roberts would know it's highly unlikely to work, and could easily backfire. But perhaps one of his clerks is less cautious (though they too would likely understand the risks).
The leaker is some other Supreme Court employee: secretary, IT person, etc. In that event, the motive could be either 1 or 2, depending on the person's ideology, or something totally idiosyncratic. I think this is highly improbable, as such people are unlikely to have good access to the opinion, and even less likely to be able to make confident judgments about the potential effects of a leak. I think this scenario is also at odds with Politico's description of the leaker as "A person familiar with the court’s deliberations."
The leaker is a hacker or some other complete outsider. I think this is even more clearly ruled out by Politico's description than 6. But I include it for the sake of completeness.
If I had to bet, I would put my money on 1 or 2 above. My guess is there's at least an 80% chance that it's one of these two possibilities. But of course, I could easily be wrong!