Poor chaff spreading which results in uneven surface moisture to seed oilseed into. If there is too much tillage, chaff rows tie up nitrogen. This results in yellow strips in crop as well as increased weed growth. Now I try to straight combine everything leaving 12 inch tall stubble to match my drill spacing and chop and evenly spread straw and chaff. If you bury weed seeds they will likely grow.
Less tillage means less weed growth.
If you doubt this look at June summerfallow that you worked in May. Compare it to one that hasn't been worked. The weeds are larger where it wasn't worked but there are less of them. Try and relate this to direct seeding. Less tillage at seeding equals less weeds in crop. My third year of barley resulted in root disease and a 10 bushel crop.