Andrew Brunson was arrested in 2016 because of being a member of FETÖ, which is an organization responsible* for the coup attempt in Turkey.
* This is the consensus in Turkey. In addition, the opposition thinks that Erdogan's party is responsible too, because of supporting FETÖ in the past (especially before 2013).
US President Donald Trump wanted Brunson to be released just a few days after the state of emergency ended. Since he was not released, the USA started to impose some sanctions on Turkey. Turkish President Recep Tayyip Erdogan said that he could be released only if the USA extradited Fethullah Gülen to Turkey.
Erdogan: They want a priest from us, and you have a priest either. Extradite him, let's judge.
I wonder why the USA is reacting so strictly against Turkey.
Is Trump doing what he would do for every US citizen, does Brunson have any other duty other than being a pastor, or is Brunson important for Trump's domestic politics in the USA?
Note: Lots of people were purged unfairly after the coup attempt, but I don't know if that was a case for Brunson. If so, that could be an answer too.
FETÖ [...] is an organization responsible for the coup attempt in Turkey.is far from being universally accepted, and there are claims that Erdogan is just using the excuse of prosecuting those responsible for the coup to get rid of political rivals.