I find myself feeling rude when I can see that any advice given is being completely discounted, and people are pouring effort into sorting out a situation whilst the OP is refusing to implement any of it.
Or if the OP has asked if she is being unreasonable and, after being informed that she is being unreasonable, then either dripfeeds the rest of the information explaining why she isn't being unreasonable, or simply shrieks "I am not, how could you think such a thing!!???"
People who denigrate a large section of society with wittering about their choices make me feel very rude ("AIBU not to let Ds1 go to a birthday party on a council estate?"), ditto people who ask what really should be obvious ("AIBU to give ds1 some dinner tonight even though my husband said he doesn't need any because he had a big breakfast and has been cheeky all day?")
People who ignore the fact that their partner is abusing/mistreating their children make me feel very rude ("DS1 says he hates his dad - his dad makes him stay in his bedroom unless he's at school")
Ignorance makes me react rudely. Passive-aggressive behavior makes me react rudely ("oh colditz, don't swear, it upsets me, I feel bullied, colditz, you're a bully!!").
I'm quite rude in general though. But hey, at least I do usually try to help. I am very rarely unconstructively rude.