Dark forest attacks all share two qualities: one, they’re casual; two, they’re economical.
By ‘casual,’ what I mean is that the only basis for the attack is the exposure of the target’s location. There will be no reconnaissance or exploration conducted against the target beforehand. For a super-civilization, such exploration is more expensive than a blind strike.
By ‘economical,’ what I mean is that the attack will employ the least expensive method: using a small, worthless projectile to trigger the destructive potential already present in the target star system.