It's hard to tell much in detail about their reasons from the article. I read it as just a change in tactics in how they move large blocks of stock. They seem to feel that, currently, moving a large amount of money using their previous algorithmic strategies has become too easy for other traders to read (perhaps also algorithmically), and then interfere with in ways that are detrimental to their trades. And they believe that negotiating large outright block trades is a better strategy in the current market, given their current goals. Maybe true, maybe not; it seems like a very fact-specific decision, hard for someone without the data to evaluate just from general principles of markets.