There are so many causes: humidity, under or overcooking the meringue, using old eggs, and refrigeration (a necessary evil) just to name a few. Luckily, cooking the meringue first before adding it to the pie does wonders for preventing weeping, which is why I prepare my pie this way. It's not 100% foolproof, but so long as you follow the steps (watch the video if you are a beginner) I'm confident you will have success, and no weeping!