this forum is like a tree with one ROOT. The ROOT has many child nodes which are new posts, each of the new post has its own child nodes (replying posts) and grandchildren (replying posts to the replying post), and so on.
However, the server has limited resources (e.g memory) or it's not capable of maintaining a big dynamic tree, so it needs to cut off the layer of the nodes at some level.
If the reply reaches some level, it will be saved as a sibling node (that's how it automatically makes a turn) instead of a child node.