I'll be blunt: Wikipedia's official policy, "Wikipedia is not a battleground", is probably the single least-honored rule in Wikipedia history.
Although it clearly states that "personal conflicts" or "ideological battles" are OK on Wikipedia, and it advises contributors to act "civilly, calmly" and cooperatively, any veteran editor will confirm these things happen with regularity.
In fact, there is a specified arena for such battles, called Administrators' noticeboard/incidents (ANI). It's either fascinating or tedious, based on your familiarity with the topic.
If against the rules, why does it happen? The simple answer is that collaboratively determining the "correct" interpretation of controversial topics naturally descends into acrimony. A more complex answer would explain why Wikipedia prioritizes content creation over civility.
The only good news: if you remain civil, it's fairly easy to avoid. More on that next week.