Business leaders used trusts and monopolies (for example) to take over competing businesses.
With a trust, multiple companies worked together to eliminate other competition. With a monopoly, one business worked towards eliminating all competition.
Once competition has been eliminated, the trust/monopoly has enough power to regulate prices on that specific good/service.
This posed a danger to society, because there were no competing companies who sold the same service/goods for less money. This forced all customers to pay a high price because they could not find other businesses who offered it for cheaper.