Why is there a special pronunciation of 论 just for 论语? The character 论 has pinyin lùn for all uses except in 论语 where it is 
lún.  Is there some known historical reason for this change in pronunciation?
After a bit of research, I've come across a possible explanation.
According to this source from the 语言文字报 Journal of Language and Writing (not sure the English name is correct) it appears that the character 论 was archaically used with the meaning of to edit, to compile as in the  word 论纂 lun2 zuan3, same as 编撰 bian1 zhuan4. The reference given is the scholar Lu Xun who in his teaching materials for a class at Xiamen University, talked about the Analects:

﹝ 孔子 ﹞ 既卒 [...]， 门人又相与辑其言行而论纂之， 谓之《论语》
[After Confucius' death], the disciples revised each other's notes [of the master's teachings] and compiled a book, which they called &quot;Edited words&quot;.

So if we take this for good, the 论 lún in 论语 is a legacy of the archaic 论纂 (to edit, to compile) as Confucius' disciples compiled his teachings into a volume that later became known as 论语, which literally means “edited words&quot;.