Facing the sun or moon when relieving oneself
Question

                What is the reasoning behind the Hanbali madhab stating that it is makruh in the to face the sun or moon when answering the call of nature ?

This is the mutamad position as it is agreed upon in both al-Iqna and al-Muntaha.

Ibn al-Qayyim states in Miftah Dar al-Sa'ada (2.205-206), "Not a single word about this has been reported from the Prophet , not with an authentic or weak isnad, or a mursal or continuous isnad. This issue has no foundation (asl) in the shar'."

But still there are still some other strands of Hanabila left in the world who hold this opinion.
Praise be to Allah.
      The view of the Hanbali madhhab is that it is makrooh to 
      face the sun or moon when relieving oneself; the reason they gave for this 
      ruling is that there is some of the light of Allah in both of them. 

    Shaykh Mansoor al-Bahooti (may Allah have mercy on him) said: 
    And it is makrooh (to face the sun or moon) without a barrier when relieving 
    oneself, because there is some of the light of Allah in both of them, and it 
    was narrated that there are angels with them and that the names of Allah, 
    may He be exalted, are written on them.

    End quote from Kashshaaf al-Qinaa‘, 1/62 

    The Hanbali view with regard to this matter is the view of 
    the majority.

    Radd al-Muhtaar ‘ala al-Durr al-Mukhtaar, 
    1/343; Haashiyat al-Bujayrami ‘ala al-Khateeb, 1/196; al-Taaj 
    wa’l-Ikleel li Mukhtasar Khaleel, 1/408; al-Mawsoo‘ah al-Fiqhiyyah, 
    34/9 

    The second opinion is that it is not makrooh to face the sun 
    or moon when relieving oneself, and this is the correct opinion. 

    Al-Nawawi (may Allah have mercy on him) said: This is the 
    favoured view, because judging something to be mustahabb requires evidence, 
    and there is no evidence with regard to this matter.

    End quote from al-Majmoo‘, 2/111 

    Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him) 
    said: 

    The correct view is that it is not makrooh, because of the 
    absence of any sound evidence and because the evidence which indicates that 
    it is permissible is proven.

    End quote from al-Sharh al-Mumti‘ by Ibn ‘Uthaymeen, 
    1/123 

    And Allah knows best.