A: Where wind for various reasons blows down the chimney preventing gases and smoke from rising up the chimney to the atmosphere.
A: This could be caused by the flue being blocked by mortar or something else try getting the chimney swept.
A: Where the pressure at the fire is less than at the chimney pot, sucking air down the chimney.
A: This is probably caused by lack of ventilation which is causing the pressure to drop in the room.