So-called "baby carrots" are actually blemished or deformed "adult carrots". The defective carrots are cut into short lengths and lathed down to remove the abnormalities. It's much more profitable than selling them as horse feed or just pureeing/mulching them.
I guess the cutting machine made an extra partial cut for some reason. The rest of it is the natural core of the carrot as Grey_Devil pointed out. Maybe hitting the slightly denser core triggered a safety halt in the cutting machine.