There are many ways this happens.
The owner dies or is incapacitated and family or friends don’t know what to do with the cats. Too often the easiest option is to just put them outside to fend for themselves.
Many businesses and trailer parks support trap-neuter-vaccinate-return (TNVR) efforts until a change in management occurs. The new manager or owner decides the cats must go. This leads to trap and dump policies or worse.
Some pet owners allow their cats to roam freely outside. People move and leave their cats behind. Some grow tired of their cat and drive it somewhere and abandon it. People, people, people.
Cats escape their home or carrier and are lost outside.
Perhaps the biggest way cats end up on the streets is through the failed animal sheltering policy called Managed Intake. This is where shelters decide to close their doors to or limit animal intake. Shelters instead use appointments for intake with the waiting list being months out. Shelters refer people to rescues to take in what the shelter has refused. Sadly, rescues are overcapacity as they try to do their work and that of the shelter refusing/limiting animals (since there are so many cats, taking in fewer of them easily raises a shelter’s Live Release Rate and makes it no-kill because there aren’t many live outcomes for the cats = nowhere to go). Many shelters take in few cats. They push the notion that cats can live just fine on the streets. Unfortunately most of the cats they divert to the streets are unfixed.
No matter how a cat ends up living on the streets, this is certain: an unfixed cat on the streets will only lead to many, many more unfixed cats on the streets.
Community Casts United, Inc.