Extremely speculative and based on nothing but pure, unadulterated dubious armchair/reddit science, but: L-theanine is a (mild) depressant/anxiolytic with reported effects not wholly unlike GABAergic substances' (benzodiazepines, pregabalin, gabapentin, phenibut, alcohol) reported effects. GABAergics modulate or increase GABA-related activity. (GABA is the primary inhibitory neurotransmitter.) L-theanine is structurally similar to glutamate (the primary excitatory neurotransmitter) and interacts with glutamate receptors in a way that inhibits their normal activity. Increasing GABA-related activity and reducing glutamate-related activity can potentially have similar "mellowing" brain effects, like the equivalency of pushing a "slow-down" button with more force and pushing a "speed-up" button with less force.

For some reason, GABAergics make some people more prone to irritability, and, in some cases, can reduce the threshold for anger and rage. Despite not being a GABAergic, I could see L-theanine potentially causing a similar effect in some people.

(To anyone with neuroscience knowledge, please feel free to address errors/leaps here.)