It can also be a sign of defiance or caution, for example when showing respect to an enemy ('You are strong and I do not trust you').Sometimes, lowering the head is just a sign of exhaustion.
Again it may be a deliberate concealment, sending covert agreement to a colleague.
When the head is low, raising it may be a sign of interest as the person moved to looking at the point of interest.
Lowering the head whilst maintaining eye contact can also be a strong flirting signal, typically by women.
It says 'You are superior and I just can't take my eyes off you'.
This is typically accompanied by other expressions of interest such as raised eyebrows.