This problem has caused an abrupt switch from meringue to whipped cream on many pies. A food science article in Fine Cooking magazine on this often-troubling, fragile confection may have a few answers. Fasten your apron strings and prepare for Meringue 101. The right amount of sugar stabilizes the egg foam, besides adding sweetness. Once you add sugar, you can beat the whites without worrying too much about overbeating.
Egg proteins tighten and squeeze out water which browns because of sugar content.
To correct weeping, it helps to pile the meringue onto a piping-hot pie. It gives a dry seal between meringue and filling. Lowering the oven temperature and increasing baking time can also help. Food gets hotter in the center when cooked on low for longer. Lowering the temperature and decreasing baking time, or both together, solves beading, but that gets tricky because you run the risk of weeping.