Once carbon begins burning in the core of a high-mass star, the outer layers begin to fall inward, driving up the fusion rates and speeding up the star's evolution primarily because

Fusion rates and star evolution increase rapidly because of the conversion of hydrogen molecules to helium.

Explanation:

At the early stage of a star mass, hydrogen atoms are produced. After a few billion years, these hydrogen atoms formed at the core of the star mass begin to fuse on the external side of the core. Hydrogen atoms get rapidly fused, resulting to a contraction of the star core, and as more hydrogen atoms get fused, helium atoms are formed.

This action results into rapid into a rapid and violent burning action of hydrogen atoms around the core.