We can cause "reactions on someone" or "reactions in someone"?

The difference depends on "in", pertaining to those things that lie inside some conceptual or physical boundary to a thing, and "on", pertaining to those things that are on or put on the exterior of the boundary (and may possibly then be transmitted onwards into the body of the thing). Let us consider the judge and the group as the two relevant "things".

The judge acts (has an effect) on the group by letting them know of her judgement; the group gets excited internally when it thinks about the judgement; their reactions to it come from within the group. They may react by shouting "Boo to the judge!". So their reaction on the judge is to shout and boo. She hears this on her exterior; whether there is any reaction within her is not known from the sentences.

The judge's judgement acts (externally heard or read) on the group and stimulates reactions (lots of animated discussion and calls for a response) in the group. The group then reacts on the judge. Consequently we say that the judgement caused reactions in the group and that the group reacted on the judge.

{I apologise if I seem to have laboured this.}