From the LA Times story:

     The attack happened at a duplex on June 7 when an exterminator climbed a
ladder to the second floor and sprayed something into an exposed pipe,
neighbors said. The man wore protective bee clothing, including a helmet and
mask, said Terry Prado, who owns a garage directly behind the building.
     Suddenly the air was filled with what Prado thought were flies.
     "They just poured out of that house like a tornado," she said. "They
just started swirling around, and this swarm just got bigger and bigger."
And then:
   What the tenants later learned was that an enormous colony of bees had
taken up residence under the floorboards of the second floor right below the
family's bathtub. Bryant's team of exterminators made the discovery when they
removed a 2-by-3-foot section of bathroom flooring to expose a vast
honeycomb.
     "They filled six trash bags full," Pena said

   Judging from the death of an animal (a dog) and the stings to multiple
people in the area, the first exterminator (not Bryant) did something wrong.
I would value the contributions of anyone who could advise on a more
skillful, less dangerous way to resolve the situation as presented in the LA
Times in the timeframe of one afternoon.

Here goes my observations:
1) He/she apparently did not take the time to diagnose what kind of insect he
was dealing with. If he had, he would have blocked all exits to prevent
exactly what happened.
2) He/she sprayed pesticide during the day, which meant that many bees would
be out in the field unaffected by the spray. A hazardous situation would be
created for at least the next few days.