Trump's been on record as disliking free trade agreements, wanting to renegotionate or pull out of NAFTA. This duty is in line with his proposed policies. The dumb thing about it is that Trump is opposed to free trade agreements because they're seen as causing the USA's negative trade balance, but the aerospace industry is actually the greatest generator of trade surplus for the US (
source), so putting trade restrictions in place for that sector (which other countries will certainly levy in kind on US products) is shooting himself in the foot.
Delta partnering up with Air France-KLM may perhaps have something to do with it as well.