Still, even if one-third of his colleagues have complained about him, I doubt they'll be willing to set the precedent of tenured profs being fired, they'll strike for him if the union requires it.
* Experimenting with eliminating grades.
This was the issue at hand anyway.
* Altering a course curriculum with student input.
Every teacher does this to some extent; there aren't enough details to judge (i.e. did he make a class on physics into one on cooking?).
* Criticism of his peers for paternalistic teaching methods.
Not enough details, but it sounds like it was restricted to emails and was therefore just words..
* Held a popular course on activism that the university cancelled the next year.
Can't fully judge from this; maybe this class was popular cause it was one where he was experimenting with giving free A's. Or maybe it was popular because it was good.
* "[...] [M]ade headlines after 10-year-old twins registered for his course with their mother – and he supported the filing of a human-rights complaint claiming ageism when the university said they couldn't stay."
Supporting their filing shouldn't be grounds for dismissal; sounds like he was just going through the appropriate channels. He didn't e.g. let them come into the class in defiance of the rule, he just supported their appeal.
* Doesn't shy away from expressing his view that Isreal commits military aggression
If expressing the opposing view can't make you lose tenure, expressing this one shouldn't either. Unless they want an Orwellian culture.
But for some reason Unions always end up answering to the lowest common denominator. So you have a group of people who largely want to do their job well being forced to walk away from that job to defend the idiots who do their jobs badly.