Probably due to fog at ATL earlier this morning (it has since cleared up but assume that rebooking is well underway).

Also - are you a high value or low value Diamond? While anecdotal, I was dealing with serious IRROPS a few months back and I got through to an agent within 2 minutes while a co-worker I was traveling with - also Diamond but barely one - called at nearly the exact same time and didn't get through for 20 minutes. Could be coincidence but I believe even within Diamonds they prioritize calls depending on whom they deem as a HVC.

Even during the worst days I've never had to wait more than 5 minutes to get answered so 15 does seem unusual.