Tomato flowers are dropping

Before some of you get excited at yet another failure, some of my tomato plants are doing well in the greenhouse, (though only about 3 toms any ripe yet). I have a Brandywaide outside that looks healthy enough but a lot of the stems are weak and breaking before the flower heads. Some are ok but I don't understand what is happening ?

Yes, but the actual flowers stems are breaking. There is no weight to cause any stress as they are still flowers.