Definition of "labialization"

These labialized stops are of course nothing but positionally determined allophones of the [t] and the [k] phoneme, and their labialization is non-distinctive

1935, Bernard Bloch, American Speech

The second question concerns the reasons for the labialization of this suffix in [1.3] - in the case of artukluk one immediately thinks of an assimilation (*u - ı > u - u)

2013, Bülent Gül, Ferruh Ağca, Faruk Gökçe, editors, Bengü Bitig. Dursun Yıldırım Armağanı‎, page 551