I can't say I know for sure, but I'm also an AT&T customer and what I believe the problem is based on bits and pieces I've picked up from research is that the path AT&T is using to get you to the SE Data Center has flaws along the way.

Some notes from my end:

Yes it tends to be most common in the evening
AT&T service is performing to Spec according to Speedtest and my livestream software during heavy in-game lag.
When using WTFast GPN all lag is eliminated. (GPN gives you an optimized route to the data center.)

It's becoming easier to believe that AT&Ts route is the issue here.