From the helpfile:

This spell allows an Invoker to channel a forking bolt of lightning into
a group of targets or similar creatures. The bolt will fork as many times
as necessary to strike all who are associated with the initial target.

However, this happened:

Victim yells 'Heelp ssoomeeoonee iiss aattaackiing mee!'
A bolt of forked lightning shoots forth from your hands!
You fade into existence.
Your forked lightning MASSACRES Victim!
The gods protect Groupmate1.
The gods protect Groupmate2.
Your forked lightning === OBLITERATES === a bystander mob!
You hear the sizzling pop of electrical arcs raging across another bystander mob!
Your forked lightning *** DEVASTATES *** another bystander mob!

Their group was even fighting the first bystander mob, and they certainly weren't "associated" with the first target in any manner. The second bystander mob was a complete outsider.

This doesn't happen when attacking a mob; the spell remains contained. If this is somehow by design, then please at least add a remark about it in the helpfile?