An egg cracked during the boiling process and expanded from within.
Can You Eat Hard-Boiled Eggs That Cracked While Cooking?
By Gregory Lovvorn
From an early age, most of us have been taught to avoid raw eggs and discard cracked eggs, as they have a high potential of transmitting the Salmonella Enteritidis bacteria. Per USDA, 2.3 million of the 46.8 billion eggs Americans eat yearly are contaminated with Salmonella, and consuming “these eggs results in a mean of 661,633 human illnesses per year."
A USDA Food Safety and Inspection Service report titled "Shell Eggs from Farm to Table" states that "if eggs crack while transporting them home from the store, the USDA advises breaking any cracked eggs into a clean container. Tightly cover the container and refrigerate it, using the eggs, within two days. If eggs crack during hard boiling, they are still safe to consume."
However, even though you should consider boiled eggs that crack during cooking safe to eat, the same USDA report notes that eggs can be contaminated with bacteria even when there are no cracks in the shell. Because of this, the USDA recommends boiling eggs until both the whites and yellows are firm and thoroughly done.