It’s hard finding a credible article with credible source that lays out why Obama fired Flynn. (forced him to retire). Flynn served as Director of the Defense Intelligence Agency, from 2012 - 2014. Flynn played a key role in counterterrorism and dismantling insurgent networks in Afghanistan and Iraq.
He became exasperated with Obama for his inaction with ISIS apparently. I read he was against Obama’s ideas to arm Syrian Kurds. (https://www.chicagotribune.com/nation-world/ct-obama-trump-islamic-state-plan-20170202-story.html)
“Planning for the final assault on Raqqa, had been grinding on for more than 7 months, There were no good options, but Obama's top foreign policy advisers were convinced that they had finally settled on an approach that could work - arming Kurdish fighters in northern Syria. There was just one problem: The Obama team had deliberated for so long that there was no time left to pull the trigger”.

Another article says Flynn was exasperated because he wanted a streamlined program to use information obtained during raids in Afghanistan, but Obama refused to give it to him. And a Breitbart article quotes a WH official saying Obama warned Trump on Nov 10th to Not hire General Flynn. https://www.breitbart.com/news/obama-warned-trump-against-hiring-flynn-before-inauguration/

The fact that Trump did the opposite and was able to eradicate ISIS within a few months proves that Flynn was right.