There could be several reasons. What I suggest below is all the opinion of a layman, so if your problem persists i'd definitely recommend talking to a doctor even if it's your general practioner, family doctor, or whatnot.
I believe (but don't know) that stress /or lack of sleep can cause spastic twitching of the eyelids.
If the throbbing occurs behind the eye, or if you have a sudden "eye ache," the problem could be eyestrain. Eyestrain can be caused by keeping your eyes focussed at the same distance for long periods of time, working under bright lights or in areas of very high contrast, changing focus often, and working near predominantly blue or purple colors and lights. (the last may seem fanciful, but it's true.)
if your work has you staring at more or less the same thing all day, you may be suffering from eyestrain. Problem scenarious could include staring at a computer screen all day, working on an assembly line and having to change focus regularly, reading from notes and then addressing an audience, and so on. Your vision is controlled by muscles, and having these muscles exercise too hard or simply remained tensed in one position for too long can be tiring.
Try relaxing a few minutes of every hour, or at least allow your eyes to focus at a different distance. If you drink a lot of caffeine, try cutting down for a little while.
If relaxation and a change of habits doesn't help after a few weeks, consult a doctor. If the problem interferes with work or with driving, consult a doctor as soon as you're able.
- gary
