What is/are the cause/s of becoming "perceiveless", an unconsciouse-being?
SN 23.2 defines "a being" ("satta") as strong clinging to one or more of the five aggregates. One the meanings of the word "satta" is "clinging". To cling requires a mind. Therefore, it appears there can be no such thing as an unconscious "being".

In my search, I only found the term asaññasattā in AN 9.24 plus the DN and, in the sequence of the teaching, appears related to the 4th jhana. The 4th jhana is obviously not "unconscious". It follows it appears the term "asaññasattā" needs to be examined carefully, for what it really means. The impression is "asaññasattā" means not having any salient perception because the mind only has equanimity as its object. It appears "asaññasattā" does not mean "unconscious". 

Regardless, these types of teachings found in the Anguttara Nikaya &amp; Digha Nikaya are often suspicious and, imo, best ignored.