It literally tells you its Graveth being asked for support, not you. In this case its your kingdom which is the aggressor in the incident therefore you don't have to do anything. Its the Rhodoks who will have to either let it slide or attack.
If it was the other way around then it'd be your dilemma. Its probably why its considered unprovoked to declare on them, because they're the ones being provoked.
The message says “local notables….have been mistreated by their overlords from Camelot (my kingdom) and petition king Graveth for protection.” Sounds like it’s my faction doing the mistreating
I’m a queen of my own faction in Mount and Blade Warband, and I get a border incident message that local notables from a village claimed by the Rhodoks have been mistreated by overlords from my faction. I know if I don’t react, I’ll lose face and relationship points with all my vassal lords...