Basically people who use the system to generate interest on their stake rather than to reward value contributions. That can take various forms, and most involve some degree of self-voting or vote-selling, but the exact scheme isn't that significant.

What is relevant here is to simply limit the number of content items that can potentially generate a high-efficiency return in this manner. If such returns can be broken up in too many pieces (without losing value to some form of superlinear curve), then downvoting that relies on mostly altruistic volunteer effort will be overwhelmed.