Not even close to an expert on Georgia politics, but I remember in 2014 there were some personality-based endorsements by Zell Miller, that might explain these as well.
Miller endorsed Nathan Deal, R, for Gov because he kept funding for Miller's crowning achievement, the HOPE Scholarship. Also, Deal, a former Democrat himself, seems to be personally friendly w/ Miller as well. And on top of that Miller and Jimmy Carter (Jason Carter was DEM Nominee for Gov in '14) never had a good relationship, politically or personally.
Miller endorsed Michelle Nunn for Senate because of his good relations with Sam Nunn.
This most certainly seems to be the case with the relationship between Barnes and Isakson. That article you posted (first link), states this:
"Barnes and Isakson have been two dominant forces in Cobb County politics since the 1980s. The former governor, who launched a comeback bid in 2010, likely gave Democrats some heartburn last year as the party searched for an Isakson challenger when he said 'if all Republicans were like Johnny, I would be a Republican.'
Barnes had this to say in a brief interview this afternoon:
'Johnny and Dianne have been friends of ours since we were young and served in the General Assembly together. I am a Democrat and I have nothing ill to say about his opponent, but Johnny is our friend.'"
Nunn also said the same thing:
“is an effective public servant and has been my friend for many years.”However, U.S. Rep. David Scott's endorsement of Isakson is setting off some alarms:
http://politics.blog.ajc.com/2016/08/04/georgia-democratic-congressman-says-hell-back-johnny-isakson/While I do think personal friendship plays a big role in some of these endorsements, I also smell something fishy about Jim Barksdale.
