If it is a dry day and you have headed all the warnings about meringue making (no yolk in the whites, no salt in the mixture, impeccably clean bowl and beater, the correct ratio of sugar to whites, and sugar added very gradually checking before each addition to see that the mixture is not gritty) it is possible that you are over-beating the whites which can break the structure of the foam but it is more likely that you are not allowing the crust to dry out long enough.
Hard meringue should have 1/4 cup sugar per egg white and should be beaten just until it is glossy and forms tall peaks that do not curl at the top. The crust is really put into the oven to dry out rather than to bake. The process starts with 1 to 1 1/2 hours in a 225°F oven. When you think the crust is thoroughly dried, insert an instant-read thermometer in the thickest part to make sure it has reached 160°F then turn off the oven and allow the crust to continue to dry out in the oven for several hours or overnight. You can find more information on meringue and meringue pie shells on the American Egg Board’s web site.