He announced it himself via social media:
After much consideration, I will be stepping back my weekly role as Smackdown Announcer to dedicate more (cont) https://t.co/An7zuYRFmV— John Layfield (@JCLayfield) September 1, 2017
The full statement:
After much consideration, I will be stepping back my weekly role as Smackdown Announcer to dedicate more continuous time on the work I have done since 2010 with at risk kids and communities. My new schedule will allow me to continue to be a part of the WWE family and also continue my work that I feel is the most important thing in my life. I will continue my 20+ year relationship with WWE and will appear on such marquee shows as Tribute to the Troops, WrestleMania, amongst others.
With the help of Beyond Sport, I founded Beyond Rugby Bermuda in 2010--which has been recognized as one of the world's best charities for work with at risk, gang focused, youth intervention. Beyond Rugby Bermuda won the 2014 NACRA Fair Play Award out of 7,000 programs and 17 countries for its work with at risk kids. I was named as a Beyond Sport Global Ambassador--an organization backed by all major sports leagues in the U.S.
I'm proud to say that in addition to being Smackdown's longest reigning WWE Champion, I inspired WWE's Tribute to the Troops over 10 years ago. WWE has been my partner in all my philanthropic works and this new deal with WWE ensures that will continue. WWE has made my dreams come true and helped me become a global name. And now, WWE is helping me use that name to make a difference.
WWE commented on as much:
WWE admires JBL’s continued determination to make a difference in the lives of at-risk kids in Bermuda and around the world.
No word on whether or not WWE will bring in another announcer to replace him or go with the two-man booth of Tom Phillips and Byron Saxton.