The herbicide 2-4-D butyl ester, if sprayed within miles and conditions are right such as an inversion, can be blown off target and kill some veggies with just vapors. Fifty years ago Dow Chemical was experimenting with herbicides in South Texas and it got off target because of an inversion and damaged crops for miles around.
2-4-D has been around for a long time. Very effective broadleaf killer. I used it regularly in fence rows and in cornfields. But you had to watch out for the wind when near alfalfa, clover, soybeans, gardens, etc.
But ‘Banvel’,... that stuff has 2-4-D and something else (don’t remember), and had a stronger scent. You could use like 2-4-D, but it worked slower, yet was more powerful. And it would linger in the air much longer, and move with even the slightest air movement.
So why did the “drift” you suggest not kill everything in the area?
I don’t see how someone managed to get into the farm to do this though or why.