In truth the revolver in question in proper working condition needs the trigger pulled back to fire. Holding the trigger back then cocking the hammer back and releasing will fire the weapon. Think of the ""fanning"" of a six shooter seen in western movie gun fights.
The possibility of a defective weapon is also a reason. The catch on the hammer from safe, load cock to full being defective or a worn action allowing for the hammer to fall unexpectedly. A very very light pull to no locking of the hammer.
Also noted the firing pin sits exposed with the hammer down. If a live round is under the hammer it can fire the round!
https://www.youtube.com/watch?v=5LigAFOlhC8
Go to around 13 minutes....