I have to answer the thread title in two ways, Min, because it is not an easy thing to pin down.
The actual trigger that made me walk away was what I perceived to be the blasphemous position claimed by the GB/FDS, they had totally shoved Jesus aside ,and claimed for themselves His position.
It suddenly struck me that ,in their constant shrill calls for "submission" and unquestioning "loyalty", they demanded actual worship !
But also what actually made me leave was my having been an Apostate my whole life, I never agreed with all the teachings, the big spur to research was the realisation that 1914 was just simply not in scripture, never mind about when Jerusalem was destroyed by Good King Neb, the whole 2520 years rubbish is not as credible as Dan Brown's Da Vinci Code!
The whole WT/JW belief system is a house of cards that soon flutters to the ground as a small breeze of Truth blows upon it.