First, they let their competitors onto their platform without charging extortionate fees. They wait for them to become large and well-used on the platform. Then they change the fee structure so that it's no longer profitable for their competitors to compete on the platform at all. This leads to customers switching apps for their needs.
It's abuse of market power from being the marketplace/platform owner.
As I always say: a market or platform owner/operator should not be allowed to also be a competitor within it. The incentives get messed up and abuse will always follow.