Chunks disappearing in a really old world

A huge set of chunks disappeared in my world. This world is quite old and nothing like this had ever happened before. If you look at the map in the image provided, you'll notice a black chunk, this patch is the corrupted patch.

Something to note is the fact that I had placed a huge number beehives and bred many bees before the occurrence. Another thing is that I had traveled to a previously unexplored area in the world (about 1000 blocks away on both the x and z axis).  A third thing is that the world took an unusually long time to load. 

I really don't know how to reproduce the bug or why it happened.