@willow11,
Funny because I remember reading that deep voices in men is more to intimidate other men rather than to attract women so I would imagine this is more intentional - meaning the man deepens his voice on purpose rather than instinctive.
That being said - and I am one woman - I love deep voices and find them very attractive - perhaps these men that do this have found that certain women like this as well so why not increase your chances by deepen your voice?