I've searched the literature on this topic and cannot find even evidence of respiratory effects from trenbolone -- but that's because it's never been the subject of study. Considerations for cattle will never encompass cardiorespiratory fitness, but rather matters that pertain to survival, mass, and transportation efficiency. Matters of study will never revolve around fitness metrics in humans because trenbolone is not legal for that purpose. Absence of evidence when the question would never be raised in the literature (i.e., cattle and their relevance to the agricultural-industrial food chain) does not prove the converse, however.
To ask for a mechanism just shocked me, because it's such a grand request. All you have at your disposal is bro science, and that alone should be given its due respect IMO!
If I were to speculate, hypothetically, about the closest thing I, Type-IIx, on a bodybuilding internet forum thinks could be construed as a "mechanism," it would be that trenbolone's potent binding to the AR (it really fits like a knife into the receptor) causes such a potent anabolic response in muscle (and some other) tissue that the organism must redirect energy away from other metabolically costly functions like the cardiorespiratory system's delivery of O2 and removal of CO2, removing waste products, et cetera to direct it to the potent muscle building functions being asked of it by the trenbolone-AR complex the writing of proteins in a metabolically costly process.