The Sumo Council has set up a body to review tournaments to see whether they can see any evidence of someone throwing a bout. The recent basho has been given a clean bill of health but the report noted that foreign wrestlers tend to speak with each other and shake hands which they think might raise suspicions of collusion. For instance, Mongolian Shotenro spoke with his compatriot Kyokutenho for about half an hour before their match. Perhaps they'd prefer to see a few backstage punch-ups.
Source (Japanese)