The Fonz wasn't allowed to wear his leather jacket when not in a scene with his motorcycle, by decree of Paramount, so the producer gave the writers the rule that the Fonz would push the motorcycle in every scene

 and he pushed his motorcycle into his apartment, into the diner, etc etc so that the Fonz was always wearing his leather jacket

Heard on an interview this morning https://www.iheart.com/podcast/731-dsc-29668130/