Why is this so mysterious? It looks exactly like you'd expect an oblique-angle impact would (and the article speculates that it is the most likely cause).
I assume that they are rare? I'm not sure if they are possible on a planet with a decent atmosphere like Earth to, given that things that shallow would be flung back into space.
Maybe the incoming body broke into several bodies before impact. Combined with an angle of attack wouldn't the smaller bodies down before larger bodies due to atmospheric drag.
