The Mecha mod set: every so often, your kubrow marks an enemy. When that enemy dies, any status effects the victim suffered are applied to its allies within a radius.
Sometimes the kubrow marks a velocipod. When you tranquilize a creature and call for pickup, the creature switches to an 'ally' (if I'm to believe the way its marker changes on the map).
The Mecha mark applies the victim's status effects to its allies.
You are the victim's ally.
Hence: radial friendly-kill aura