Why are they angry, Tep?
Because they've cross-bred with the Africanized bees, which are not-totally-unjustly labelled the "killer bees." When our friend first got his bees (literally in the mail; the post office called and said "You have a box here for pickup...it's full of bees."), they were from some bee guy (a bee farm? a bee breeder?) who can somehow guarantee their provenance. Apparently they were Italian, or maybe Russian. Seriously.
Anyway, the Africanized bees have penetrated this area pretty thoroughly, and there's no way to keep your fancy Italian bee colony from cross-breeding. So after the first year, his bees had cross-bred with the Africanized ones.
The example he gave was that, the first year (pre-Africanized cross-breeding), he could mow the lawn near the hive and they wouldn't give a shit; they'd just kind of hang out on their front porch and watch him mow. And when he checked on the hive/honey production, he would put on his bee hat and gloves, but not a full bee-suit, because they pretty much didn't care if he took the top off the hive box and checked things out.
Now (after 2 or 3 years), he doesn't mow near the hives, and when he goes to check out the hives he wears his full bee suit and duct tapes around his wrists and ankles so bees don't get in, because when he goes near the hives, they come after him, like some kind of horror movie.
They're just angry little bees with issues, man.