Unresolvable conflict
What exact event is a trigger of schizophrenia? Depression? Mania? Catatonia? It seems like psychiatrists tend to use extremely vague wordings in their answers. Let's make things clear.
We need to break the main control flow of the system and force it to go against itself by feeding data to it. What vectors of attacks are possible and can lead directly to these diseases?
Tricky sequences passed to the pattern-recognition modules, equivalents of code injections, make you misinterpret data from your inputs. It's weird, but hardly damages the system.
Memory has to be updated using new data, which leads to an awkward feeling of cognitive dissonance. It's not a disease.
In a busy environment, your memory caches can be overwhelmed by information. It's like a DoS attack of sorts. Makes you distracted and unproductive. In some cases, cache overflow can be abused to push unprotected data into the processor, increasing suggestibility so to speak, but it's not a permanent disease.
Hallucinations and dissociative disorders, caused by reading and writing into memory during its hot maintenance, are important issues, but not the ones we're looking for.
Dangerous environments and related trauma push you towards suitable patterns of behavior. Our system is designed to handle this. It won't break completely. What else can we do?
There are conflicts of interest. Limited resources force you to evaluate priorities and make hard choices. It's stressful, but it's not enough. We need to orchestrate an unresolvable conflict inside the decision-making unit in order to break it.
Provide a choice with equally valuable options that'll require a subject to use logically incompatible patterns of behavior of class I, mirrored for schizophrenia, arbitrary for catatonia, proactive or reactive for mania and depression respectively.
This kind of choice creates hesitation, almost catatonia-like stupor or agitation that can't be resolved by reasoning. We apply serious tension that rips apart two main switches that control the direction of behavior, and the forces aren't "just thoughts", they are real conflicting physical processes.
Switches won't last forever under such abuse, especially if they're already compromised by bad genes aka wrong assembly instructions, disruptions during early development, or various processes that just wear them over time.
Unresolvable logical conflict can become the last drop.