Q: Why does the status date of a submission change periodically though the status remains unchanged?

From your question, it seems that the status date of your submission has been changing from time to time, while the "current status" has not changed. Your status has been "under review" for a long time, with the status date being updated from time to time. While there's no way to be certain about this, my assumption is that the status date changes each time reviewer comments are added to the database and/or whenever the editor checks this database. However, the status will not change to "review completed" unless the editor has received all the reviewer comments. Thus, it seems that some of the reviewer comments have come in and the editor has started accessing the database to view the comments.

I know what a stressful time this can be, and the obsessive need to constantly check updates makes it worse, but because it’s been only a month since you submitted your paper, I suggest you be patient and wait for the reviews to be completed, after which  the status should change to “review completed” or “in decision.” Good luck!

Another submission-related query most authors have is What is the meaning of "decision in process" status? and how long they should await for the journal's decision.

You will also find this ebook useful: 
Keep calm and wait: A guide to understanding journal statuses