This. Any pesticide sprayed on a nectar source that would have this kind of quick action would have killed/disabled the foragers before they made it back to the hive.
For this kind of effect the pesticide would have needed to be directly applied. Any evidence of anything being sprayed into the entrance? Oily residue or some dead vegetation around the entrance? This could suggest someone hit it with wasp freeze or something. The can can shoot 15 feet.