I had a seedling do that once also.
ALL seedlings start by sending the root up first, then it makes a 180 degree turn.
This is true of ALL seedlings.
They do this to create the natural bend to force the shoot to shed the casing.
There are many, many pics and diagrams on botany sites (not RIU).
If the bean is planted shallow, and the shoot gets hung up on a large piece of perlite or a hard clump of dirt or medium, the seedling can bend the other way and the root pop up instead of the seedling. Not by a defiance of nature, but because the seedling is bent like a spring, and the blockage is causing the tension to push the other end up.
I found one like this a couple months back and was able to actually re-plant it. It was under a dome and the environment was humid and the seedling newly emerged, so it lived.
It is flowering as we speak.
EVERY TIME you plant a bean, the root heads up before down.