Often what causes this is when somebody attacks a mob, but for whatever reason doesn't kill it-- either they died fighting it, or simply found it was too powerful for them and ran away-- the mob continues to wander about until someone does finally kill it. Then it re-spawns and waits quietly for someone else to come along.
So really best thing to do is run away yourself if you cannot fight it, or call out for help.
I'm happy to take care of an out of control rogue, but only if it is _VERY_ clear that you want my help-- I'd hate to 'rescue' you from a rogue only to find I've stolen your kill.
As Bartholin pointed out, sometimes if you are standing within a certain radius of a freshly spawned mob, that can cause the mob to attack too, so if you see someone fighting a rogue, stay very clear.
