Terpenoids are compounds that, besides cannabinoids, are the major constituents of the resin, and give it its flavour and fragrance. They are basically the aromatic oils in hash. The reason they're harsh is that you're condensing them from a gas to a liquid on the inside of your lungs, which is obviously tickly. Having a little look at my books though, apparently it's the most volatile terpenoids that cause the most coughing, so I really have no idea why the vapour gets harsher towards the end of the hit, sorry. I would have guessed that it was an adulterant with a fairly high boiling point, but you say this is really good piece. I'm stumped, hopefully someone will be able to answer that one for you.