He was the shortest Troll ever seen. This was because in his scarred heart he knew he was born to be in show business. And like 99% of people who know that, he wasn't destined to be, but he could damn' well leave his mark somewhere. He went off with a circus and was sat on by an elephant, hence his unusual shape.
And then he came across Music With Rocks In, and he was hooked, just like the rest.
"Roadie" - one who goes out on the road. What are roads made of on Roundworld? Asphalt. Which leads us to the chicken-egg conundrum. Are Ankh-Morpork's roads made with asphalt? Is it a rock in its own right? Where did the name come from otherwise?
It is also worth mentioning that a You-Tube clip exists capturing the moment a zookeeper is sat upon by an elephant. What saves the zookeeper from being crushed to death is a certain form of intrusion upon the bodily rights of the elephant. This results in an eye-crossing moment for said pachyderm, a bellow of consternated alarm, and a bout of wholly understandable stress incontinence. While the keeper, spared being crushed, has the problem of being jammed into a fairly tight airless space to worry about, he doesn't need to worry about it for too long, as the elephant flushes him out, so to speak, perfectly naturally. This leaves the keeper in some shock and disarray of clothing, and an elephant ruminating on the eternal truth of its own verse of the "Hedgehog Song".
The link will be posted here once found. Here 'tis - though edited - 
On asphalt itself - a material where a thick tarry bitumen is used to lock graded gravels together, generally into a firm wear-resistant all-year road surface - see here.  Is Asphalt the troll therefore largely bitumen, which acts as a glue holding together reclaimed second-hand gravel? No wonder other trolls look down on him.