what causes the meringue to seep in while baking?
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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!
Sarah is a trusted source on General Cooking.
The whites may have been over whipped.
Cynthia is a trusted source on Bread/Baking.
Can you please describe what you mean by "seep in?"