Have you ever thought about film “trailers”? The general dictionary definition is something like “a series of short extracts from a moving picture, a film or a movie, introduced at a previous performance to promote interest in the forthcoming production. An advertisement”. And, over the years we’ve accepted this.
All the other definitions in the dictionary refer to something behind, left behind, pulled behind etc, but never in advance. Even a pathway, a trail, is something left by passing feet.
Could it be that one of the staff in the early days in Hollywood wanted to lay a path or a trail to the next super production? Or could it be that the person concerned, and remember that many of the production staff were newly arrived European immigrants, and perhaps “ignorants” as well, and miss- spelled trial as trail, hence a trial became a trail, and a trialer, to try out the audience reaction became, by usage, a trailer?