My hypothesis is that Nextdoor primarily appeals to those who already have that kind of territorial "they're all out to get us" mindset since territoriality is literally what the app is about. It's the angry "get off my lawn" old man of social networks and it attracts exactly those kinds of people. Mix in some Internet depersonalization effects and you get something pretty nasty.
My neighborhood is 50%+ non English Speaking Chinese. There are posts almost everyday that say things like..
"Some people in this neighborhood need to OPEN THEIR EYES and stop hitting the gate with their cars. I'll post this in the appropriate languages so everyone and the community can appreciate the significance of my message. Ching-Chang Chow... Bang Bong, Bing bong bing."