I haven't really been clear on why Special Counsel Robert Mueller is prosecuting former Trump campaign manager Paul Manafort, but a February 28 article from Talking Points Memo (referencing a piece from Bloomberg dated the same day) puts things into sharper focus.
The short version is this: In the early years of this decade Paul Manafort's main client was Viktor Yanukovych, a pro-Russian Ukrainian politician who become president of Ukraine in 2010. Manafort advised Yanukovych on politics and campaigning, but he also used his influence in the U.S. on behalf of what was then Ukraine's pro-Putin government and hence at least indirectly acting on behalf of Russia. Manafort is now facing charges of having failed to register as a foreign agent in the U.S.
Yanukovich was ousted in 2014, sending him fleeing into exile in Russia. The new government charged him with treason and accused him of corruption as well. Russia staged military incursions into eastern Ukraine and seized the Crimean peninsula by force, and in response Russia was hit with international sanctions. I won't even try to summarize the details here, but with respect to Manafort, the relevant point is that with his client out of power, he was suddenly without his main source of income.
Complicating matters further, in late 2014 Manafort's family discovered that he had been carrying on an affair, and in 2015 he apparently suffered a kind of emotional crisis, reportedly telling his daughter that he was contemplating suicide, and eventually he checked into an inpatient facility for treatment.
Only a few weeks later, Manafort sought a job in the Trump campaign. In late March of 2016 he was hired, and two months later he was named campaign manager, though according to the campaign he was an unpaid volunteer.
As the TPM article points out, it made sense for Manafort to try to reestablish himself in American politics, but it's also true that he had long ties to pro-Russian interests, that Russia was very interested in having sanctions lifted, and that Manafort was facing personal and financial problems at the time he took the unpaid job.
The aforementioned Bloomberg article quotes former federal prosecutor Peter Zeidenberg as saying, "Manafort looks like he's in dire straits. Then he drops everything to work for free for Trump. It's very strange." This is only suggestive, but it might explain why Mueller is concentrating on Manafort and his associates.