IIRC it's the oxides of tren which cause a prostagladin release shortly after injection causing your airways to constrict and have an asthma like attack
or you inject into/near a vein and make an oil embolism but that'd cause a cough+much more with annnyyy oil based injection.

If you cook your tren right and don't overheat it so it goes brown-red/rusty gold colour then no cough
If you overheat it and your tren turns rusty-gold then tren cough is far more likely. MTS tren has always been a sort of bright yellow-gold colour and never had cough... only had my most recent vial of tren enth be a darker gold colour but it isn't rusty-gold/red like a lot of tren is. Don't get cough with that vial either.

Funny thing is that if the oxide hypothesis is correct then people are worried about not having oxides of tren in their tren ace/enth...oxides of tren don't do anything to help you in BBing/to build muscle IIRC. They just give you coughing.