If a body is at rest and it suddenly separates into fragments for internal reasons, then the principle of conservation of linear momentum will still hold because there are still no external forces that modify it.

As a consequence of  this principle of conservation, if a person standing on ice, for example, throws a snowball in one direction he cannot help moving in the opposite direction. Likewise, if a rifle shoots a projectile, the rifle is pushed the opposite way (unless we hold it tight and resist the backward motion).  

In the atomic world, the principle of conservation of linear momentum is also responsible for the following phenomena: if an atomic nucleus disintegrates emitting a particle, the rest of the nucleus must move in the opposite direction.

In the following visual we will see the laws that govern the separation of a body into two fragments.