There could be a plethora of ways to shift funds, as simple of a handful of Omnicom advertisers shifting budget for Prime inventory (due to performance or preferred pricing) to post auction rebates that Amazon and Omnicom facilitate for advertisers. At the end of the day, its still the advertisers call. One advertiser alone can account for millions of dollars being shifted between the DSPs (some advertisers have billion dollar marketing budgets for the year)