Cohen was in financial trouble.
TheDeep State started investigating ALL Trump connections, up to several degrees of separation, as soon as he won.
They picked up where HRC's oppo research team left off.
Through Cohen's connecgions, they found a Russian immigrant who had legal troubles:
Evgeny Naumovich Freidman ("Gene") was hassled by all sorts of governmental organizations, not just the feds. A Chicago judge apparantly put him in jail last December, and possibly through that got something on Cohen to flip him.
See several posts here(with further links)
https://theconservativetreehouse.com/2018/07/24/cnn-obtains-trump-cohen-recording-from-lanny-davis/comment-page-3/