Not usually related to rust or corrosion. By fault of design, end of nut will expand inside pan when tightened too much, forced against taper to seal. They don't take much to overtorque. Some did come from factory stripped, but only those from about '90 on. Volvo issued bulliten to just pull drain plug for regular service (didn't want to pay for pans/tubes any more under warranty I believe). Also most people use a poor fitting open end wrench that can distort nut to an eliptical shape.
If they are frozen or stuck, most techs will pull tube with pan or leave on when doing trans r&r. Hard part is top starter bolt, have to lower trans crossmember to get it out. Then treat it as a unit unless it's leaking, then you will have to replace them both. Don't overtorque!