I think the reason why some topics get moved to the PDA section or merged with an existing PDA is that the general tone of the message is negative or sad or personal. We had a long discussion about this a while ago and that is where this decision was made by Admin and the moderators.
For a while it got very morbid with all the personal tragedies and the PDA section is set up to post personal "stuff." General Chit Chat is set up for positive topics or topics that potentially affect everyone.