Jade Wang from the Google My Business Maps team announced in the Google Business Help forums that if business owners do not login to their Google My Business account and do not respond to requests from Google, Google may unverify those accounts.
So make sure you login to Google My Business frequently enough to not lead to seeing your verified Google business listing removed.
In some cases, we may contact Google My Business users via email to confirm that they are still actively managing a business page. If a user is unresponsive to our attempts to contact him or her and has not logged into Google My Business for a significant length of time, then we may unverify pages in the account. We're doing this in order to continue to provide users with the best experience when they’re looking for local businesses like yours. If you find that a page in your account has been incorrectly unverified, please contact support to get assistance restoring verification.
It's a good idea to keep an eye on the inbox associated with your Google My Business (Locations) account. It's also a good idea to regularly log into Google My Business (Locations) to confirm that your business information is current and accurate.
I suspect, over time, many businesses will find themselves unverified when they should not have been.
Forum discussion at Google Business Help.