"A little more than three weeks ago, a Falcon 9 rocket launched from Florida's Kennedy Space Center carrying a payload of 60 Starlink satellites into low Earth orbit. It was the first of four such missions flown this month by SpaceX.

The entire mission was nominal, except for a problem with the rocket's second stage. Typically, within an orbit or two of launching, the Falcon 9 rocket's Merlin vacuum engine will relight and nudge the second stage downward so that it harmlessly re-enters Earth's atmosphere into the Pacific Ocean.

However, there was not enough propellant after this launch to ignite the Merlin engine and complete the burn."

Also, the 2nd stage of the Falcon is only 4 metric tons, one-fifth the mass of the LM-5 core stage:

"On Thursday night, three weeks after launching, this second stage re-entered the thicker part of Earth's atmosphere and, in doing so over populated areas, provided some spectacular views for the Portland and Washington state region. Videos shared on various social media networks showed what appeared to be an invasion by an alien armada.

This was, however, simply the Falcon 9 rocket's second stage—which weighs about four metric tons unfueled—breaking apart in the atmosphere and burning up. It is likely that the second stage completely broke apart over the Rocky Mountains near the United States and Canadian border."

Even for a 4 ton 2nd stage, SpaceX designed the capability to have it dumped in a controlled manner into the Pacific. Yet for a 20 ton core stage with a far higher chance of leaving intact pieces that survive reentry, CALT did not bother to design the margin into the core stage to have it de-orbit in a controlled manner.