The reason why they do this is not known (from what I know), but it is said that one reason might be that they are shedding and want to stretch the skin or they got something in the eyes. It might also be a sign of stress. If your dragon only does this once in a while it is nothing to worry about - it is completely normal. If it happens a lot - you should find out why.
I have a page about dragons. You can read (and see pics of the eyebulging) if you like:
http://www.morion.com/teo/english/experience/frame.html
(Look for "Behaviors" at the left frameborder. The text and picture about eye bulging is quite far down. 
/Gunilla
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