Much like the pilot’s seat within a jet fighter, the human heart is capable of ejecting from the body entirely should an emergency arise. When the mechanism is triggered, pressure builds within its chambers until a critical level is reached, at which point the aorta detaches from the rest of the circulatory system and serves as an escape thruster. The entire supply of blood within the body is repurposed towards this launch, causing the heart to exit through the mouth at just under a bullet’s speed.
Of course, losing a heart in this manner almost always results in death for the rest of the body. The organ cannot survive on its own either; its function is entirely dependent upon its connection to the nervous and respiratory systems. For this reason, it seems odd to suggest that such a thing is an evolutionary adaptation; though creatures like the sea cucumber are known to expel their own guts in self-defense, they are also capable of regenerating that which is lost. Humans have never known such luxury.
Two possible origins have been posed for this vestigial capability. The first is that it was once a defense mechanism, and that after firing, the body would enter a period of hibernation during which a replacement heart was grown. While not particularly damaging to its target, the projectile heart could still frighten off predators, as well as allow the threatened human to effectively play dead. The recovering body would have been entirely helpless, however, and would therefore require the care of a family or tribe.
The other hypothesis is more radical- that human beings were once mollusk-like in nature. Similar to a bivalve’s shell, the rest of the body was an elaborate exterior constructed around an otherwise simple organism- the heart. In such a hypothetical past, the heart was much more capable as a standalone lifeform, complete with carnelian antennae and a pseudopod to help it find its way. If such cardiovascular autonomy ever existed, it was eventually lost to time as the heart and its body grew more interdependent.
Whichever theory one believes, the mechanism still exists, and is occasionally triggered involuntarily in times of extreme stress. When the unconscious mind thinks that it is faced with insurmountable doom, the muscles of the chest suddenly tighten, and an inner countdown to launch begins.