The film's official novelisation gives a nice technical description of the incident. In short, the star destroyers behind the Supremacy were hit with highly accelerated plasma consisting of the remains of the Raddus' and its "hyperspace tunnel" (rather than, as might be supposed, shrapnel from the Supremacy).
But with the fail-safes offline and the overrides activated, the
proximity alerts were ignored. When the heavy cruiser plowed into the
Supremacy’s broad flying wing, the force of the impact was at least
three orders of magnitude greater than anything the Raddus’s inertial
dampeners were rated to handle. The protective field they generated
failed immediately, but the heavy cruiser’s augmented experimental
shields remained intact for a moment longer before the unimaginable
force of the impact converted the Raddus into a column of plasma that
consumed itself.
However, the Raddus had also accelerated to nearly the speed of light
at the point of that catastrophic impact—and the column of plasma it
became was hotter than a sun and intensely magnetized. This plasma was
then hurled into hyperspace along a tunnel opened by the null
quantum-field generator—a tunnel that collapsed as quickly as it had
been opened.
Both the column of plasma and the hyperspace tunnel were gone in far
less than an eyeblink, but that was long enough to rip through the
Supremacy’s hull from bow to stern, tear a ragged hole in a string of
Star Destroyers flying in formation with it, and finally wink out of
existence in empty space thousands of kilometers beyond the First
Order task force.
The Last Jedi: Official Novelisation