Regarding Horford, I think Al was a bit piqued when Ainge didn't give him some help up front in the stretch when Baynes went down. Horford is not a dominant player, and he had his hands full without up front help. Everybody thought Ainge was holding his chips for Davis, but he apparently changed his mind. Obviously, Tatum would have sealed the Davis deal, but he backed off on Tatum, and the Lakers found themselves competing with themselves--and the Pelicans who didn't have to trade him pre-season.
Ainge suddenly found himself short of bigs, especially with the other good teams stacking up on them. So he went out and got Kanter (good sign if he plays to his strengths inside) and Poirier (an unknown by NBA standards) and Tacko Fall ( a very tall shot in the dark). We'll see how all this plays out.