Before Camp Zen, Tenma worked as a debt collector for his Loan Shark father. Judging by Eisuke's familiarity of him & his brother, I can guess that he made a reputation for himself, likely through various, unintentional means.

Well, one of his father's targeted clients had a bright idea to eliminate him first via explosion : Tenma's apartment, blown to disproportions. It didn't work, of course. It just made his " debt reminder " proportionately more gruesome.