For the same reason cheesecakes weep: because they over-bake and the custard curdles. In other words, the egg proteins in the filling get too hot and start to tighten up into clumps. As they tighten they squeeze out moisture, causing the pie to weep. The filling takes on a grainy texture, and if the process goes on too long, it starts to shrink. Wherever the firmer overcooked spots meet the softer medium-cooked spots, cracks can appear.
The solution is to watch the pie careful as it bakes to make sure you only bake it to the point that it’s done. It’s also a good idea to calibrating your oven before you set out to bake a custard to make sure it isn’t running hot. Granted pumpkin pies are much sturdier custards than either traditional custards or cheesecakes — that’s why they can bake at higher temperatures — but they can definitely be curdled if that high heat goes on for too long.
Jiggling the pie is usually good enough to determine how the baking is proceeding. The truly obsessed can take their pie’s temperature. About ten minutes before the baking is done, insert a quick-read thermometer in the very center. You want the center to be at least 140, no more than 150.