In 1987, Marcus traveled America with a friend. Two years later, while rollerblading on California's Venice Beach, he was discovered by a photographer. Schenkenberg (who had been earning money as a dogwalker and a nanny for two young children) agreed to have his picture taken and within months he was en route to the runways of Paris. His career really took off when Bruce Weber shot him showering naked for a Calvin Klein jeans ad.
I cannot believe how these supermodels are always 'discovered' in the most strange ways...I wonder if it is true or is just a 'story' they make up to make it sound like they were not really interested in modeling.
https://www.supermodelguide.com/supermodels/marcus.html
He is well built so I am sure that is what drove the interest. As for making up stories, I doubt it, a lot of photographers (usually called scouts) are on the lookout in everyday life for 'just the right' person or face based on what they know the clients are willing to pay. So finding him while roller blading does not sound very fanciful really.