The news changed afterwards.

Only one was a doctor. One was the director and one an accountant. A pharmacist was injured.

They were Christian missionaries who were there to heal and convert (for a good analogy, imagine if there were health care clinics here in the US set up by Sufi Muslims to dispense meds AND the Muslim religion. We are shocked... but many of the people there did not like the foreigners coming in with a foreign religion.)

The Yemenis claim to have arrested the murderer. Posters may recall a somewhat similar incident in the Lebanon last month when an American missionary nurse was murdered.
Whether it's a specifically anti-American thing (perhaps not unrelated to the recent"drone"-killings) or an anti-Christian thing is unclear.
I suspect some of both.