I do extracurricular lessons with schools - teaching engineering through fun experiments and the like. I'm not very deep into the teaching sphere as such, but I think close enough to make some anecdotal observations. Frankly, I have seen some pretty disgusting practices from teachers I've worked with. Not to say that is the majority, but it only takes a handful of poor teachers to muddy the water. Three theories as to why that is happens:
Firstly, many teachers subconsciously have a tendency to see their classes as a group entity, not as a selection of individuals. As such, when a couple of kids behave badly in class they abstract that feeling of "getting attacked" to the whole group, and lash out defensively against everyone in said group. The amount of times I've seen a teacher unfairly punish a kid because a different one has irritated them over the course of the lesson is astonishing.
Secondly, from my experience it seems that the type of people that go into the profession (thinking more pre/middle school ages, not high school/university) often have a propensity towards narcissism. I don't know why, but this is just what I've observed. They have little power outside of the classroom, had little power growing up, and were never really especially gifted academically; but now they've been given power over large group of kids with little to no means to respond. I see a lot of teachers projecting the problems they have/had at home or with the school bureaucracy onto their classes, and use them as a metaphorical punching bag for their internal struggles.
Thirdly, teaching work at lower ages has sunk from a middle class job to a middle-lower class job. The stagnant wages has meant that a lot of talent has been put off teaching where we need it the most, and with that has gone some of the better educated. Some of the children I work with are honestly better at some subjects than their teachers are (thank Khan Academy, YouTube educators, et al for that!) and some teachers do not appreciate it. Just last month I saw a teacher shoot down a child for correcting him, even though he was not only right, but polite about it. I was sure to let the administration know what I thought of this behaviour...
Note; please don't take this commentary as me condemning teachers. The overwhelmingly vast majority of them are incredible people that in my eyes are poorly compensated for the service they do - but like I said earlier, it only takes a couple of bad eggs to put some kids off of education for life, and that dissolution translates to the class rooms of teachers who are truly amazing at what they do, which ruins their experience with teaching too.