From the Wikipedia article, it sounds like a combination of factors. Huge number of birds, potential landing gear issue, and very short time between emergency and impact.

I see some armchair analysis over on reddit (https://old.reddit.com/r/aviation/comments/1hoky8v/video_of_jeju_air_flight_2216_shortly_before/) but really we’ll have to wait for the investigation to know exactly what happened.

Was it a Boeing plane?