People who live in a half-dozen homes along a private lake in the Dallas suburb of Irving, Texas, have lost much of their back yards. They woke up yesterday to find that an area about 50 yards long and ten yards wide had sunk about six feet. Nobody was hurt, but the cave-in destroyed decks, stairs, birdhouses and fences. The cause of the collapse remains under investigation. The lake is a water-filled former quarry. The homes were built in the mid-80's.