Market maker model (MM)
Brokers of this type are usually among the larger ones on the scene. In order to act as a counterparty to all their clients' trades, they need to have a really high level of liquidity. However, this could lend them a fair amount of motivation to meddle with the trading results of their clients. If it is a proven broker without a dark past, there is probably no reason to worry. However, there are known cases where even larger brokerage firms have artificially increased slippages, set minimum stop loss intervals, or influenced their clients' transactions in other similar ways.
The reason for this behavior is quite clear. In the MM model, all losing client trades go back to the broker (not to the interbank market, where they would end up in other broker operating models). Thus, brokers built on the MM model may have a vested interest in the loss-making performance of their own clients.
Figure 1: Schematic of the MM broker's operation