While this phenomenon is rare now-a-days, Fluffsplosion refers to a pregnant fluffy dam becoming chemically unstable and exploding, killing the mare and sending its foals, blood guts and other body parts flying in all directions.
This is usually caused by the mare being under enormous stress while near full swell or just for comic relief.
More often in stories, mares may have a spontaneous Fluffsplosion if they are greatly surprised by a distressing source or object, say a dog barking directly at it or a clattering of pans falling out of the cupboard.
Some cannons blame the explosion on a chemical process, whereas the mare's muscular system clenches so hard that the overflow of amniotic fluid ruptures in all directions, mixing with the other organs and their fluids and causing a fast reacting expansion of gas.
Some cannons attribute a thermal explosion, mostly caused by a unicorn mare igniting it's own Fluffsplosion.
A mare falling a great distance can also end in a fluffsplosion if she is not immediately killed by the fall, taking only a second to register the surprise and pain before coating the room in foals and entrails.