Basically, the "True the Vote" folks were able to get "big data." Cell phone tracks, voter registration records, etc... They're for sale...
Then, they winnowed things down until they saw patterns... People who hadn't voted in a decade suddenly doing absentee ballots, and not just a few. They got video (FOIA type availability) from dropbox sites, etc...
Seems that there were a lot of people who would visit a ballot aggregation site (such as a local campaign HQ), and then drive by 20-30 dropboxes, dumping a few into each.
Some of them took pictures, presumably to prove that they were doing so.
I suspect that it will be even more spread out next election...