It's not just any reaction. Your reaction to seeing a flower is not the same as your reaction to rocks falling on your head. To call this a "reaction" would be to pointedly understate what is occurring.
There are different words that could have been used, like "alarm" for instance. But to leave it at "reaction" would be leaving out important details of the story that could otherwise be conveyed in a single word better than just "to react".
There are different words that could have been used, like "alarm" for instance. But to leave it at "reaction" would be leaving out important details of the story that could otherwise be conveyed in a single word better than just "to react".