CP-22A's are often related to changes resulting from info provided by taxpayers.  Perhaps your clients have had some interactions with the IRS recently?

I have seen at least one CP-22A that had a very brief explanation on the first page.  If we hadn't prepared that amended return, we may not be able to make the head and tail of what that's about.

Agree it's probably the most expedient to call the PPS to find out what caused the increase in tax if your clients had not been in contact with the IRS (or received any prior notice).

The simplest way to submit a PoA these days is via the online service but the good old fashioned way of submitting PoA's with wet signatures by fax still works:

https://www.irs.gov/tax-professionals/submit-forms-2848-and-8821-online