Africanized and many black bees can become aggressive over 30k.
Outside of the rare and occassional warm/hot hive i find that on my area there is a time when population exceeds forage and this forced scale down of brood rearing makes them a bit more cranky. They know the happy days of unlimited forage are done, this creates stress, stress opens up disease spread. So a variety of things are happening. That can make them seem aggressive.
More bees more mites
Big population low forage
Superceduring
Too hot too cold too wet too dry
Some forage like some golden rods will turn pussycats to lions
Helicopter beekers
Yellow jackets and robbers
Mite treatments like formic and hop guard
I once had an africanized swarm take over a colony and you will never forget it, if it happens to you.
Most people have a low threshold for defensive bees.
If your bees are too defensive look at larger picture in equipment, forage, time of year mutes disease etc.
Nothing says I love you like buying a girl a drink....give some syrup and a few days see how they are.
I will often during dearth periods throw a pail of syrup a few hundred feet away from hives to lure all nectar foragers away from hives, and this makes things a lot easier during naturally defensive times.
Breeding from more gentle stock can help a lot but your not in a vacum so hopefully your neighbors are good beekers.
Moderator's note. The practice in bold is common in the USA. It is frowned upon and definitely NOT recommended in the UK