When people ask the question "why?" it often makes people being asked the question defensive, yet an inquiry about "how did you come to that decision?", or "what lead you to that decision?" allows people to share without offense.
Really, test it...look in the mirror and ask a "why" question, and watch your face distort and hear your voice accuse. After that, stay in the mirror and ask a "how" or "what" question, and see the softness in your voice and face.
Keep your communication direct and without a wrongly-perceived attack by resisting "why" questions and replacing them with the "what" or "how" varieties.