Well a synonym for "to film" is "to shoot." You can "shoot/film" a movie. I guess if you are filming a turkey, you aren't exactly filming anything interesting. A bad movie would probably be one that isn't interesting.
Turkeys, as in the birds, have long had a reputation for being foolish or stupid--though the show business use of "turkey" apparently predates the fool/moron use of the word.
The Online Etymology Dictionary suggests that "turkey," to mean a movie of low quality and especially one that is a commercial failure, to 1927.
Movies are "shot" just like photographs are shot: point at your objective and press a button. I'd guess that there was a happy union of these two phrases, "shoot" and "turkey" when people began to make movies.