The insinuation here is the obsessive behavior. The text calls it a madness so I’m guessing it was a sudden occurrence.

I’m not an expert on Autism or other mental situations like ADHD mind you but it seems pretty likely Thrasyllus became hyper fixated on boats one day and he basically became a harbor manager for it much like you see other hobbyists become conductors or train line engineers for toy trains and that before the very modern diagnosis of Autism came about these people were considered strange until the behavior could in some part be explained by neurodivergent behaviors/tendencies.