Normally a Runner Bean produces one stem which is the clear "leader" - like this:-
In the axil between the main stem and each leaf a small shoot grows, but it normally remains insignificant unless the main shoot is pinched-out or damaged. In fact some growers advocate deliberately pinching-out the growing-point in order to stimulate the production of side-shoots. I generally only pinch-out my Runners when the stem reaches the top of the cane.
Sometimes the leading shoot runs into trouble, like this one. It has just stopped growing for some reason (pest-related, maybe). It has gone brown and is beginning to shrivel.
However, when this happens, the plant responds by switching its energy to the secondary shoots, ones which would normally remain insignificant. It's the same principle as cutting the main head of a Broccoli plant to stimulate the formation of side-shoots. That's what has happened here. The two shoots next to the bottom pair of leaves have put on a burst of growth.
In this next shot you can see the original main shoot wrapped round the cane at the right, whilst a second one is racing to catch up, and thrusting out strongly towards the cane.
This particular plant won't win the race to the top of the canes, but it may very well turn out to be the most productive plant of the batch.
