When a player is in a minecart, chunks stop loading - when travelling along a minecart track that leaves the currently loaded chunks, upon reaching the loaded chunk boundary, the minecart stops and the player cannot get out of the minecart without using the /tp command. Upon leaving the cart, chunks usually begin loading again, but occasionally they won't and the player won't be able to move into unloaded chunks. Also, occasionally, pre-loaded chunks in the direction that the player comes from will become invisible, and only entities will appear, as seen in the screenshot.