It is normal for a pizza crust to become soggy with time after baking, but there are some things that will hasten the transition from crispy to soggy.
1) Pizzas that are baked at a very high temperature may only develop a very thin crusty layer on the bottom of the pizza. This can quickly go from crispy to soggy. Parbaked pizzas seldom ever experience this problem.
2) Dough weight is incorrect for the size/type of pizza you're making. Typically a pizza skin that is stretched too thin, especially across the center section will crisp during baking but loses that crisp very quickly. This is normally accompanied by a gum line.
3) Excessive use of sauce and/or toppings, or sauce watering out.
4) Failure to place the pizza on a screen so it can steam off immediately after baking without the steam being driven back into the pizza as it would if placed onto a serving tray or other solid tray immediately after baking.
5) If the problem is due to excessive oil, look at the cheese and/or pepperoni, as either of these have a reputation for potential to oil out. Some cheeses and pepperoni exhibit much greater resistance to oiling out than others.
There are probably a few more that aren't coming to mind right now, but these are what I think might be the most common causes.
Tom Lehmann/The Dough Doctor