Okay .. heres the theories .. the follow up is to come
1) it is caused by BA ,, which would explain why Al got it from Prop .. but here is the problem with that : it can't stand up to experiment. You can still get the cough even without any BA in the mix (it's been tried before by a couple of people). Any way there is less BA in tren then in most prop mixes. (if it was BA everything IP made would give it to you) By process of elimination it has to be the fina. (Or the fina combined with a certain solvent or something.)
2) The fina causes a prostoglandin release which triggers your lungs to start the coughing ... yet to be researched fully
Thanks to Cj wolf , Sky, Backon , and Superchicken