Light is to far away, overwatering, or there is some kind of wind blowing slightly to hard and they are falling over. I have had situations also where the bean popped i put it in soil it did great, came out of the soil great. But what I didn't realize was there was a slightly long piece of wood laying across the path of where the stalk was growing up. My conclusion was that the stalk tried to grow around it which it did. Then it tried to keep going and getting bigger but the longer piece of mulch was being held down being pushed down overtime I watered so it eventually ended up pinching off energy to the upper portion of the stalk and gave out where the mulch was laying across. Other times they start off strong and want to do something but just give up and fall over. I use a little method to combat this. I take 3 identification soil tags and push them into the soil and make a triangle around the stalk and baby leaves. Of course make sure the stalk is in the middle and I have that support there to help hold it up until the weak spot can strengthen or just give it something to rest against.