[Mobhack]

Was it somebody who had been shot at that returned fire?

If a unit is fired upon, its opfire filter is changed to the range of the nasty enemy unit that fired on it plus 1 hex, otherwise your units would sit dumb to any shooting at it and probably die. So if shot at, opfire filters all expand to the firers range (and armour class etc, for the CD full opfire filtering).

If you shoot a unit with filters applied - the same rule applies. Otherwise the unit would refuse to fire and end users might complain about that, or having to be told to individually turn off filtering if they wanted to fire the unit(s) in question individually.