Q: Why has the status of my revised manuscript changed several times?

You haven’t mentioned what the status update was between the two Under Review updates. Anyhow, this doesn’t seem to be a matter of much concern. It’s probably due to an occasional error. At the most, the Associate Editor (AE) may have picked it up for sending it to the reviewers again. Perhaps the two updates means that the first time, they weren’t able to send to both (or more) reviewers, and now, they have. Again, not much to worry about.

Now, you just need to wait for the reviewer’s comments/recommendations on the revised manuscript, based on which first the AE and then the Editor-in-Chief (EIC) will make a decision on the manuscript. For/Until that, you may find it useful to go through the following resources:

• Who takes the final decision on a manuscript: the AE or EiC?
• How a journal editor reaches a decision on a manuscript after peer review

All the best for your manuscript!