Well, maybe some of the plants eat the animals and this is what they need armor for!
A plant that aggressive belongs on the animal encounter table, regardless of classification. Carnivore Trapper, perhaps.
Evolution tends to work to fill gaps and exploit resources as a natural consequence. If the plant life feeds upon the animals actively, then those animals that successfully defend themselves will live longer. That defense can be sharper horns to cut themselves loose, nimbler feet to avoid the traps, sharper eyes (or noses, or minds) to recognize the danger, or a change in diet to be able to survive in a place where the aggressor plants don't live. If that other place lacks something else important, then further adaption may take place. Say there is a vital nutrient missing, but those grazers that have taken to eating a specific shrub are getting it. IF this nutrient isn't in the shrub, but instead comes from the bugs that live on the undersides of the leaves, the large grazer has taken steps to becoming an omnivore, and may take further steps into carnivory down the line. In evolutionary terms, you'll have carnivores soon enough.
But what of the aggressive plants? If some of their herbivorous food source remains in their range, things won't change quickly if at all. If all the previous food animals move away, the plants will either find a new prey animal or go back to living off of sunshine and soil nutrients. Or die.
At the same time, some of the animals that stay will have learned to exist despite the aggressor plants, and some may eventually learn to exist by exploiting the aggressor plants, either by "stealing" their prey or feeding directly on them. Here too you have the potential to regain carnivores, as perhaps some small and agile critter has figured out that the soil underneath a feeder pod is rich with iron, calcium, and "exotic" proteins from where the pod dumps the indigestable material. This can lead to a critter than will investigate the feeder pods directly, perhaps developing a method to share the meal being digested, steal chunks off of a meal that is still being dragged into a feeder pod, or even exist within the feeder pods, immune to its processes, and feed off of secondary digestion products. Carnivores and scavengers are the end result here (to the extent that evolution ever has an end result).
Given enough time, every exploitable niche in an ecosystem will be exploited, often by multiple parties that may or may not recognize each other as competition (think lions and hyenas and vultures vs the maggots that finish the job). It never stops.
