There are some irresponsible people who at some point in their life decide to get a pet. They don't think it through in order to realize whether they are indeed ready for such a responsible step, taking responsibility for a whole life.
Some of them get an animal as a birthday gift to their children. Play with few months and then throw it away till the kid is fed up with it. Some of them want a fluffy cute toy to play with while they have the time without realizing that it's not a toy, it's a life and has needs. After a certain time, they come up with an excuse to get rid of it. They either out of a sudden get allergy, or get pregnant, or move to another city/ conuntry and can't take it with them, or or or. A number of excuses.
While they are trying to get rid of it, they understand that nobody wants their used already grown-up "toy". Everybody wants a new cute small one to most probably do the same. They also realize that the shelters won't help them and accept it since they are full already. Most of such people don't even care to offer to shelter to cover the monthly expenses of the animal they are getting rid of. They just want to pass through the responsibility.
Once they realize it's hard to find a new place for the animal, they just throw it out of their house. Without considering, that the animal is not used to surviving on the streets. It's a house pet. There are certainly people who won't even try to find a new house for the animal, they will just throw it away on the spot.
If the animal manages to survive for a certain period of time, maybe a few months, it most probably gets the opportunity to consummate. Simple math. Two animals being thrown away produce 4-6 more animals. Each of the last ones will also produce 4-6. And so on.
(The article will be continued till the end of June)