Shopping
Unfortunatly Kashruth has become big business and there is alot of money involved. Dont kid yourself, money can and does corrupt. It has been a problem in kashruth since hechshers started appearing,
Because the OU is the largest name in kashruth , they have the most to lose if there was a problem. Many smaller hesherim do not have that name and if a problem happens, they can just close shop and re-open (and this does happen unfortunatly)