what are they bombing yemen for?

Bush and Obama did it too. Historically, it's been a targeted killing thing against Al-Qaeda (or so they have said), with whatever government they have, giving their blessing. If other sites are correct, Trump did it more, but it's kinda hard to pick nits there.

https://en.wikipedia.org/wiki/Drone_strikes_in_Yemen

That's why a lot people are more upset over the lack of operational security than the action itself. They're not conducting themselves in a way that keeps our country safe, They skirting monitoring and can't even get that right.