Dead Canadian thistle in a corn crop

An organic farm in Pennsylvania had challenges with Canadian thistle in one pasture section. One-fourth of the farm was in intensively managed rotationally grazed pastures for a five to seven-year period before shifting to other crops, and then going back to pasture.

After the pasture sod was plowed down,  biological amendments were applied and tilled in, and corn was planted. The corn and thistles both grew quickly. Cultivation cleaned out the thistles between the rows, but those in the row escaped. When I visited the fields in mid-summer, the crop was approaching the dent stage, and the Canadian thistle that survived the cultivator were all dead. Samples of the dead plants were sent to a lab to identify if any usual pathogens caused the weeds to die, but no known pathogens were identified.

The corn crop went on to make 32+ tons per acre of silage.

This field is now back in pasture for several years, and the thistles have not returned. Do you have any idea why they may have died?