A change in volume settings between the supply and return fans for large buildings that have supply and return fans. If the tracking is not set up properly or there are other mechanical problems with sensors, motors, dampers, could be quite a few things this may cause exhausting to be less then supply volume causing positive pressure.
If you have a building with roof tops then there could be a number of different scenarios depending on the building set up. Exhaust dampers could be not working properly, causing improper exhaust. This would cause the building to pressurize. You would notice other symptoms as well like possibly doors that do not close properly and if you had a building with elevators then you may notice a lot of air movement around the doors.
I guess if the positive pressure was on only one floor then this could be a blockage with air movement between the floors, or if if you had a supply duct let go for that floor there could be more supply volume then return within the plenum for that floor and may cause the positive pressure.
Would also want to know if the positive pressure was taking place all the time or from time to time.
Hope this helps