https://www.tabletmag.com/sections/news/articles/russiagate-obama-iran
This is a pretty good write-up.
What it comes down to, is Flynn disagreed vastly with Obama's foreign policy decisions to align against what America considered long-time allies, and towards what America considered long-time terrorist-state enemies.
Flynn was vocal about his disagreement... therefore Obama considered him a traitor and hence spying.
If it were reversed and Obama were a Republican... Flynn would be called a "Whistleblower" with protected status and he would be called to Testify in all kinds of hearings about his horrors witnessing the President trample the Constitution.
Edited by North co-co champs at 16:09:23 on 05/21/20