It started in 1870 where Russell attended a presentation by Jonas Wendell, who had been a Millerite.
In 1872, Russell met George Stetson at a church where he pastored. George Stetson was part of the Advent Christian Church (and had worked with George Storrs, another Adventists that was influential on Russell). Russell’s bond with George Stetson was great enough that he actually conducted George Stetson’s funeral.
Then in 1876, Russell read one of Nelson Barbour's magazines Herald of the Morning that talked about 1874 being the invisible presence of Christ, ect, ect. He met with them and was bought the whole thing, including Barbour’s prediction for 1914.
Barbour predicted that the rapture would be in 1878. Russell bought into it so much, he sold all five of his clothing stores, and backed Barbour’s publications.
Of course, the 1878 failure caused Russell to rethink things, and by 1879 broke out on his own with Zion’s Watch Tower and Herald of Christ’s Presence. Of course, he retained the whole thing about 1874 and 1914, and retooled 1878 into something invisible.
I believe this is classic sunk cost fallacy. He had sold everything around believing the belief that the rapture was coming in 1878. Therefore, a failure meant that he rallied around that belief system.
When Russell started his own magazine, he was anti-organization and the umbrella of Watchtower was big enough to accommodate varying viewpoints, including even Trinitarians. That changed after 1881.
He evidently became a successful writer with the help of his wife Maria. But by the 1890’s, Russell seemed a very different person than he was when he started out. And by the end of his life, he had a following who regarded him the Faithful Slave and the Laocedean Messenger. As he died on a train in Texas, he asked to be dressed in a toga, and that is how he passed away.