As an author and editor of journal papers, there is nothing worse than reviewers that accept to review a paper, propose a major revision and then decline to review the revised version of the paper. This is the worse thing you could do. If you accept to review a paper you’re committing to stick to it until a final decision has been made. Otherwise, you’re forcing the editor to invite another reviewer in the middle of the review process.
The new reviewer may have a completely different view on the paper. And there is nothing good coming out of this. For instance, it could generate a full new round of reviews since she’ll have new comments different from those sent to the authors in the first round (and therefore not addressed by them) or even force a rejection of the paper if she dislikes it completely (again, for different reasons than those signaled by the original reviewers) making the effort put by authors in the first major revision to waste.
This has happened to me as an editor but also as an author (in a paper that ended up rejected when after the first major review we got a second major review with a brand new set of comments, even some of them contradicting those we got in the first round).