All permabans are for consistently toxic behavior after a large number of warnings and suspensions of play through time bans. Whether verbal abuse is the most common reported offense as a correlation among permabanned players is a fact or not, I could not tell you. I do see verbal abuse a lot when I do the Tribunal, also I see offensive language and negative attitude being reported a lot. I'm not sure which is the most used to make a report, nor do I feel it completely matters as much, unless it is intentional feeding, and then I try to look for any signs of it.
They got reported for a reason, and my job as a Tribunal judge is to determine whether or not their behavior deserves to remain in the community. Sometimes it is unwarranted, and I choose to pardon, but it is generally a toxic player.