According to the Dungeon Master's Guide (page 24):
When a creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes to abide on the plane where the creature's deity resides. If the creature didn't worship a deity, its soul departs to the plane corresponding to its alignment.
In addition, it is also possible for a soul to be claimed by a powerful fiend. For example the definition of Hades states (page 63):
Hades is the destination of many souls that are unclaimed by the gods of the Upper Planes or the fiendish rulers of the Lower Planes.
This statement is complemented by a number of references about the archdevils striking deals with mortals, in the section on the Nine Hells. For example, about Dispater we read (page 64):
He is more obsessed than most devils with striking deals with mortals in exchange for their souls ...
Based on these we can conclude:
- Souls worshipping deities whose realms are in the Nine Hells
- Souls claimed by archdevils because of some deal
- Lawful evil souls that are not claimed by any deities or fiends
Additionally, an unofficial Forgotten-Realms-specific answer by Ed Greenwood can be found in a series of tweets on March 1, 2020, answering the question of how Faerûnian souls (not collected by deities) might end up in the Nine Hells, aside from Faustian bargains.
Mortal souls from Toril end up in the Nine Hells because of curses, because of the bargains you mention (sometimes through wagers lost), and sometimes because of other mortal actions taken in life, such as magical rituals that unintentionally bind those carrying out the ritual to a particular archdevil, or other traps set by devils to snare the unwary. Even careless summonings may allow a devil to claim the summoner’s soul in future payment, or a miscast spell may make an ensnarement possible. The ways are surprisingly many, so the supply of souls into the Nine Hells is fairly constant, if not widely noticed in lore.