In the scene where Judy and Nick visit Mr. Big's driver Mr. Manchas who
just after telling them about Mr. Otterton, a florist who had disappeared, who kept screaming about the Night Howlers before doing the same.
Later, we find out what Night Howlers are: a toxic flower that has interesting properties: animals who ingest it go savage.
The antagonists are using that flower to make it look like all predators are going wild, using a dart gun. From Nick's act (and the fact that the antagonists aren't surprised about the speed of the "effect") we know that the dart acts within a few seconds, and as soon as it hits, it starts acting, causing the animal to behave in a way comparable to Toothless in How To Train Your Dragon 2 (about a minute before Stoic's death), wimpering and rolling on the floor. This does not leave Mr. Otterton time to try to warn Manchas.
So why does he shout about them, if he hasn't been hit yet?