Why did rat deaths increase the risk of plague spreading to people?

When an epizootic outbreak of plague kills off a population of rodents, the risk of the disease spreading to humans increases significantly because the fleas that were feeding on the infected animals are suddenly left without a host.1

In their search for a new source of blood, these displaced, infected fleas begin to infest humans, transmitting the bacterium Yersinia pestis through their bites. Historically, this phenomenon was frequently observed in urban environments where large numbers of rats lived in close proximity to human habitations; as the rats died from the plague, the fleas migrated to people, leading to human outbreaks.213