Based on anecdotal evidence of event companies this is a widespread risk in the industry. For example  suppliers making prior arrangements in what appears to be free competition. This is a procurement risk that can reduce the quality and increase the cost of the goods and services.
As with all elements of corruption, it can also open the door to extortion, bribery and fraud.  Collusion in procurement is so common around the world and involves such  large amounts of money , particularly with development projects, that the information is readily available.

Other types of collusion in events include anything involving competitions and judges such as: guessing competitions,  beauty contests, music and film awards. Perhaps the most interesting collusion that was exposed and studied in detail occurred in the Sumo wrestling events in Japan. In this case mathematical analysis uncovered patterns of match fixing.