Tren cough is caused by the trenbolone being irritating to certain tissues including the lungs. The reason it will sometimes give you a cough is like Dolf said...pierce a small vein and some of the compound enters the bloodstream, from there it travels to the lungs slightly irritating them and making a person feel the need to cough. During cycles the blood concentration isn't ever high enough to cause this reaction but when introduced to the bloodstream directly it will. It isn't going to hurt you or anything...just kinda bad luck catching a small vein and have to ride it out for a minute. The reason some guys never get it and some always seem to...luck of the draw, or pin location rather.
Underground labs produce Tren Enanthate as 75g/ml or 100mg/ml and it is often recommended that first-time users choose the faster acting acetate over the enanthate version. It is recommended that injects be given to the novice user every other day. For more advanced users, injects of 500mg per week offer the best effects. However, because trenbolone has a negative effect on the libido, it is recommended that users also take testosterone and sometimes metsterolone to avoid issues with their libido. Tren Enanthate should not be used as the only anabolic in a bodybuilder’s cycle due to the limiting of mass gains when this occurs. Instead, the drug should be stacked with Dianabol, Anadrol or testosterone for maximum effect. Another reason for the reduce gains using Tren Enanthate alone is that estradol levels fall too low which causes natural testosterone to be suppressed.