The most common way that a European honey bee hive will become Africanized is through crossbreeding during a new queen's mating flight.

Why do bees become Africanized?

The Africanized bee is a hybrid species of the Western honey bee. These so-called “killer” bees were established when bees from southern Africa and local Brazilian honey bees mated. ... Then, in 1990, the first permanent Africanized bee colonies arrived in Texas from Mexico.

What turns bees into killer bees?

But killer bees—hybrids of the relatively docile European strain of honey bee and a more aggressive African relative—are particularly fierce. The hybrids emerged after African bees were imported to Brazil in the 1950s. ... They also collected bees that remained in the hive.