To be hoist by your own petard simply means to be hurt by the trap you’ve set for another. This phenomenon is mentioned in the Bible and by Shakespeare (Hamlet), too. King David said it best… and quite succinctly, he made a pit, and digged it, and is fallen into the ditch which he made. (Psalm 7:15 KJV).
Olive and Brandon elope to escape the madness in their respective alcohol-driven homes. They go away to a big city where they land up in a mostly industrialised area. An old couple on whose door they knock, seeking place to sleep, takes them in on trust.
A business tycoon visits them and gives the Field’s an offer for their home that they cannot refuse. The property is perfectly placed for a business the man plans to start there. The Fields will not hear of the Mann’s ‘deserting’ them just because they’re rich.