From Can a Laser Printer Cartridge Be Overfilled?:
...the key is to not overfill the cartridge, as this may result in toner escaping from the cartridge into your printer and causing damage.
Leakage
An overfilled cartridge leaks toner out of its refill opening onto the printer's drum, transfer roller, or belt and its fuser, causing printouts to be stained with toner. The loose toner can also travel via any rollers inside of the printer and contaminate any paper present in the printer's paper trays.
Toner Cartridges
Toner leaking from overfilled cartridges can also contaminate other toner cartridges, resulting in images and words outputting with the wrong colors. You must then remove and replace/refill the contaminated cartridges to correct the issue.
Cartridge Seals
Overfilling a toner cartridge can also damage the cartridge's internal seals and cause them to leak over time, even after the excess toner has leaked out. The seals inside of a cartridge are not user-replaceable. If you notice the cartridge leaking toner from an area other than the component that comes in contact with the drum or the refill opening, one or more seals are leaking.
Toner is constantly turned over and "tilled" with internal mechanisms that even out the toner. It must be a "loose" fill it can't be "packed" tight when the chamber is closed.