As for the explanation behind the reunion, there’s a wordy explanation (just like the explanation for the break-up back in ’98) on their official site. Besides personal reasons, they claim:
We never did “The shape of punk to come” [sic] justice back when it came out, too tangled up in petty internal bickering to really focus on the job. And suddenly there’s this possibility to do it like it was intended. We wanna do it over, do it right. For the people who’ve kept the music alive through the years, but also for our own sakes.
So get excited for this.
Oh, and by the way: ATDI are also playing Coachella. (Full list over at Coachella’s site, via Pitchfork.)
Get REALLY excited.