What were the reasons for this activity then? 1) To completely eviscerate a divided Relublican Party. 2) Give cover for a "tough stance towards Russia" by Hillary, perhaps even up to a NATO skirmish with Russia over Crimea and Ukraine. 3) hmm...
They thought they would win, but they needed something to really smear Trump, and they thought the Russian thing might do it. Manafort had some connections, and this was just the way in, selling it to the FISA court as a national security issue. When they were rebuffed by the courts, they found a new narrative to try after the DNC and Podesta leaks appeared. This time they got their warrant.
I think this was always meant as an October/November surprise. They never thought they would lose, hence the willingness to use the federal intelligence apparatus. In their arrogance, they saw this as low risk, high reward. It turned out to be the the opposite.