Are they listening/Interested?

I'm a teacher, with a class of pretty high level, motivated college students. I have a good rapport with the small group of students, on the whole, and the topic generally seems interesting (I surveyed them on it).

But...

On occasions when I'm explaining or expanding on a topic, many or most students will not be looking at me or even vaguely in my direction, but rather eyes down in their groups of four (tables in group style).

I find it very freaky, to be honest, so...

What does this mean?
a. I'm boring.
b. They are boring.
c. They are concentrating.
d. They are sleeping, shut up.
e. I should retire.
f. Any of the above.
g. It's a cultural thing (ok, but enlighten me...)

Any hints or advice appreciated.

They might not be interested in what you are trying to explaine which can also mean they are bored. In any cae, if I were in your place I would ask them directly and politely.

h. Their neck hurts.

But I agree with Ikuyo Kuruyo. You should ask them, politely and humourously as your post. And while you're at it, you should mention that it's not very polite to be not looking at the person who is speaking unless you are too busy taking notes. At least that's what high school teachers in Japan are supposed to preach.

I assume you are instructing your students in English correct?

Well we have many Japanese MD & PhD candidates here in my department with same difficulty. We find students from Japan have strong writing skills but very low in vocalizing and listening; especially the variable in structuring the sentence confuses them.

Our study indicate when students look away or glance at his peer or glance downward, normally those students are giving you a message of not comprehending you.