The clue to this association has been evident over the last fifteen years as the big players in pesticide chemistry bought up dozens of plant breeding and seed producing companies. Not the household names of the UK garden market, which are essentially 'small fry', but the huge North American seed companies developing and selling seed to Mid West and prairie farmers growing soyabean, wheat, and maize. In Europe substantial seed companies in France, Germany and the UK offering oilseed rape, sunflower, sugar beet and cereals have been snapped up. You may still be asking what is the connection. The connection is that the pesticide companies want to sell more of their agrochemicals and especially herbicides.
Herbicides are chemicals designed to kill weeds.
A weed is simply a green plant growing in the wrong place at the wrong time. Because herbicides kill living green plant tissue they must be used very selectively and carefully in the crop situation. For this reason they can be used in cereal crops to kill broad-leaf weeds without harming the wheat or barley. But most others are not selective and therefore must be carefully timed to hit the weeds before the crop seeds germinate in the soil. This obviously restricts their use.