what causes the meringue to seep in while baking?
I'd say moisture generally, by seeping in do you mean it's soggy, or just sunken in the middle? If it's sunken, do not fret, that's totally normal. I leave the oven closed till the meringue's done, and the door open as it cools. Don't worry about cracks!
The whites may have been over whipped.
Can you please describe what you mean by "seep in?"