The WTS moved to Brooklyn in 1908 just after the first split from the WTS by PSL Johnson regarding Russell's change in the New Covenant teaching. I wonder how many members they lost at this time and how many groups in Pittsburgh were loyal to Johnson? Also Maria Russell's divorce from Charles Taze Russell was finalized in 1908.
chap. 5 p. 59 Proclaiming the Lord’s Return (1870-1914)***
As the newspaper preaching gained momentum, the Bible Students looked for another location from which to originate the sermons. Why? The Bible House in Allegheny had become too small. It was also thought that if Russell’s sermons emanated from a larger, better-known city, it would result in the publication of the sermons in more newspapers. But which city? The Watch Tower of December 15, 1908, explained: “Altogether we concluded, after seeking Divine guidance, that Brooklyn, N.Y., with a large population of the middle class, and known as ‘The City of Churches,’ would, for these reasons, be our most suitable center for the harvest work during the few remaining years.”
In 1908, therefore, several representatives of the Watch Tower Society, including its legal counsel, Joseph F. Rutherford, were sent to New York City. Their objective? To secure property that C. T. Russell had located on an earlier trip. They purchased the old “Plymouth Bethel,” located at 13-17 Hicks Street, Brooklyn. It had served as a mission structure for the nearby Plymouth Congregational Church, where Henry Ward Beecher once served as pastor. The Society’s representatives also purchased Beecher’s former residence, a four-story brownstone at 124 Columbia Heights, a few blocks away.
A group known as the Free Bible Students broke off the Russell's group.
See also: Free Bible Students
In 1905 Paul S. L. Johnson, one of the traveling "Pilgrim" speakers and a former Lutheran minister, pointed out to Russell that his doctrines on the New Covenant had undergone a complete reversal: until 1880 he had taught that the New Covenant would be inaugurated only after the last of the 144,000 anointed Christians had been taken to heaven,  but since 1881 he had written that it was already in force.   Russell reconsidered the question and in January 1907 wrote several Watch Tower articles reaffirming his 1880 position—that "the new covenant belongs exclusively to the coming age"  —adding that the church had no mediator, but that Christ was the "advocate". He also taught that Christians making up the 144,000 would join Christ as a "joint heir" and assistant mediator during the millennium. 
On October 24, 1909 former Watch Tower Society secretary-treasurer E.C. Henninges, who was by then the Australian branch manager based in Melbourne , wrote Russell an open letter of protest trying to persuade him to abandon the teaching, and calling on Bible Students to examine its legitimacy. When Russell refused, Henninges and most of the Melbourne congregation left Russell's movement to form the New Covenant Fellowship. Hundreds of the estimated 10,000 U.S. Bible Students also left, including pilgrim M. L. McPhail, a member of the Chicago Bible Students, and A. E. Williamson of Brooklyn, forming the New Covenant Believers.   The group, which informally referred to members as Free Bible Students, published The Kingdom Scribe magazine until 1975. The group is currently known as the Berean Bible Students Church, with fewer than 200 members.