As a person who has dished out (and had deleted) many negative feedbacks, I have noticed the following three as the most prototypical reasons for deletion:
1) Not factual
2) Not game related
3) Retaliatory feedback
One may see the occasional "trivial" as well. The following are other "reasons for deletion" I have recieved:
*Foul language
*trivial for missing one turn
*trivial, not a valid reason for feedback
*Fishing for positive feedback
*Retaliatory and not factual
*Fishing for feedback
*Trivial for missing 2 turns
*not a valid reason for feedback
I would be interested to hear any other funny ones people have seen or recieved.