When a minecart goes off a track, it sinks almost entirely into the ground. When the cart is broken, the occupant falls down and can suffocate underground. In the image provided, the villager is in a submerged minecart. When the cart was broken, the villager fell into the ground and turned black. His pathfinding led him to eventually jump up and escape and in this case he did not suffocate, but suffocation is a common occurrence and the player needs to break the blocks to save the mob.