It could be for a breach of the Logan Act. Until Trump was actually sworn into office, Flynn technically counted as a civilian and it would be illegal for him to discuss matters like lifting sanctions without authorization from the sitting President.
It might also be the Turkish payments thing if he’s failed to disclose them properly.
Either way it’s a sham. The Logan Act is a law from like 1799 that I don’t think has ever been used to prosecute anyone. This is just a way to spin up pressure on him to try and get him to squeal on someone else.
Now I am hearing that the whole “Immunity” request is Fake Newz. What a topsy turvy world these days.