It seems pretty clear, in my opinion, what Woelfy is saying regarding rank and tone.
Knight to Duke OK:
"Your Grace, It is my honest opinion that you are doing more harm than good. I would even go so far as to respectfully suggest that the realm would benefit if Your grace would step down as ruler."
Anyone to Anyone NOT OK:
"You are being stupid FirstName. Your actions thus far show that you are as smart as yesterdays fecal deposit. I demand you give up the thrown, step down as duke and take a bath."
It is all in word choice and tone. Regardless, any noble should consider who they are speaking to when writing a message. There are no rules against any character holding a grudge or developing a negative opinion of someone based upon their correspondence. Of course, I have no idea what the letters in this scenario look like but it is clear that some feel that there was a general lack of respect in a few.