What are the biggest factors that have historically led to overcrowding in County animal shelters?

A lot has changed in the last few years in animal management across the United States. During COVID there was a spike in adoptions because many people were home. That trend continued for a couple of years, but as people returned to work their ability to care for their animals was impacted and shelter numbers started to rise. Returning to physical work and lost years of spay/neuter activities are the main cause of the current spike in animals. These factors in addition to a rise in informal “backyard” breeding has led to a situation where nearly all public shelters are over capacity. 

Related questions