1) about 1/2 of them are due to plain ol' User Error (not that there isn't room to improve the "UI" so that happens less) like not signing, not including required parts, simply leaving the ballot out of the envelope entirely(!), and so on,
2) 1/4 are "signature didn't match" (most of those are probably false negatives and should have counted, I'd bet, but given other issues I wouldn't be surprised if people in the same household accidentally signing one another's ballots or something else silly like that is some measurable part of the problem) and,
3) 1/4 are other :-/
[EDIT] looks like most of the "other" category is probably a couple towns in one county having a vote so screwed-up that 3,200 ballots were rejected en masse. That's about 20% of the rejected ballots, or about 2% of the total vote, right there.