WHY DOES THE COOLANT IN THE RESERVOIR GET HOT AND BOIL OVER AFTER A LONG TRIP

The engine may be overheating. If the coolant temperature sensor is not working properly, the actual temperature may not be showing on the display.

also check or replace radiator cap..

Above answers are correct. In Florida make sure the radiator cap will hold proper amount of pressure, also make sure you have the 50/50 mix of coolant. I'm assuming this is a 3.0 U engine. These are known for the water pump impellers to wear away. Also make sure the 2 small hoses going to the expansion tank are not clogged.

POSS BAD RADIATOR CAP

I recently fixed this problem on my Sable. The coolant reservoir had a crack and would leak only after I had shut off the car and pressure built up. I had this problem for a long time, since it didn't leak very often and would quit when the coolant got low enough in the reservoir. I too always thought it was boiling over... bought a new radiator cap...no change. Finally caught it leaking out the crack... changed the reservoir...problem solved.