eye movement during meditation
If you feel alert, and not sleepy or distracted while it happens, it may be one of the things that naturally occurs as you start to get deeper concentration. 

A number of vaguely unpleasant experiences can spring up- aches, itching, etc.; they come with the territory, and aren't problems in and of themselves, although they may feel frustrating when you're starting to enjoy the sense of concentration.

Take note that it happens, and take note of the feelings of frustration or distraction as they emerge. If you have time, see if you can sit until you get to the other side of it; but don't worry too much if you can't, right away. It's likely a sign that you're doing well, believe it or not.