This happens because all the moisture from the pie goes to the bottom crust. Here are sone hints:

Brush the bottom crust with beaten eggwhite before filling. It cooks and seals the crust

Sprinkle a little flour or instant tapioca into the fruit to absorb some of the juice.

Cook with the rack nearer the bottom of the soven.

If it is a one crust pie, prebake for 10 minutes, as you would a thin crust pizza crust, before filling

Partially precook the filling to rduce moisture and cooking time.