Can someone tell me what is going on between the US and Yemen?

Iranian Proxy war. Iranians are testing to see how far we can be pushed because they saw earlier Putin getting away with buzzing our ships and threatening to shoot down our aircraft as well as bombing our troop positions in Syria. Iranians bolstered by the fact they are getting the upper hand in Syria after Lebanon fell to them earlier. Third former Arab country to wobble and fall into Iran's grip. Domino theory revisited with Saudi Arabia on the list but not engaged yet. Pretty sure they will attempt a take down of Jordan next.

It is a proxy war between the Saudis and the Iranians... and the US is backing the Saudis.

There is a considerable amount of internal strife that exists in the country, and the US until 2014 had partnered with the Yemen government to prevent the spread of terrorism. The Houthis, bolstered by Iran, overthrew the government and are now basically state sponsored terrorists. Our presence is a small thorn in the side of a now radical government that would love nothing more to inflict casualties on the US.