Earlier quoted context omitted.
My guess was over something like this communication: https://web.archive.org/web/20220817182407/https://www.bette... Guy seems like a bit of a dick but I suppose after quite a few years trying to effect change in an org most people get that way.
If this is the worst, this speaks a lot more about the Thunderbird project leaders than about this person. I read this as a thoughtful, passionate, plea for project (governance) improvements. You can certainly disagree with him (I don't enough much about the project to judge either way, although his arguments ring true to my general software development instincts), but he was not attacking anyone or being a jerk.
On the other hand, the exact terminology used in the details of the ban here[0] do lean toward the less severe side of CoC violations. It seems likely that it was death-by-a-thousand-papercuts (frequent & persistent small violations coupled with an evident unwillingness to accept feedback), rather than one big blowout incident as is more often seen with such bans.