Rotten/eaten stems of broad beans

I sowed some broad beans at the end of winter and they've been doing well, and were just setting flowers. However, quite a few have rotten or eaten stems which have gone brown and rotted.  Looks like they've split and caused the plant to put up new stems. I've pulled up the rotten ones and sown new seeds, but can anyone suggest what has caused this? I can only find chocolate spot mentioned as the main pest/problem.

It could be.  For clarity, the brown bit is the problem part. The picture is taken after I'd snapped the stem off pulling them up, so the green parts that are cut are just what I've done!!!

Roots were fine. It is like something has sliced the stem open vertically and from that they have gone brown and fallen over around the wound.