Mystery Root Cluster

I was growing a few drought tolerant beans all around my veg garden. around an area that wasn't heavily planted I realized that the plant was doing terribly compared to the others. I decided to dig it up because it was so stunted. Upon digging I happened upon the horrific truth, it seemed as though the plant had been suffocated by TONS and TONS of small strong white roots which had no obvious origin. Theres some cape ivy and blue morning glory which as taken over the area about 2 meters away from the plant and theres an orange tree on my neighbors side of this concrete wall about a meter away from the plant. Any ideas as to what the culprit is?

Both ivy and morning glory are infamous for their spreading roots.  However, you say drought conditions.  If you have been watering, and your neighbor has not, his orange tree may have sniffed out your water, and diverted its roots your way.  Severe all of the roots (but leave them there to decompose), and watch to see if his tree dies over the next several weeks.  Encourage him to give his tree a deep soaking.