The monster is filled with tiny symbiotic parasites, which are themselves the larval form of a different creature. These creatures are usually benign and dormant, acting as a large scale immune system of sorts, picking less benign parasites out of the monster’s hide, digestive system and other bodily cavities. However when their host dies they immediately go into a beserk feeding frenzy, gorging and growing in size terrifyingly quickly. These ravenous larvae tear apart not only their host but any other creature unlucky enough to be within chomping distance before beginning a rapid metamorphosis into a tiny yet locust like adult phase.
The adult phase then swarms, feeds, breeds, disperses million of eggs on the breeze and dies. The eggs find new hosts and perpetuate the cycle. This symbiotic relationship evolved as a good way for the symbiotes to be sure of a high energy food source to fuel their breeding phase, while the monster benefited from improved health during the larval phase and having a ‘suicide bomb’ available to deter predators.
The side effect is, of course, that the larvae utterly demolish the monster upon death. Nothing is left at the end bar the husks of millions of symbiotes that have since transformed into a ravening swarm of death.
Oh, and the ravening swarm of death itself...