Arc flash calculations assume the door is open.  This arc flash video shows why.  Although not an extreme amount of short circuit current for this test, it is enough to create enough blast pressure from the arc flash and blow the door open.  Do the doors blow open for every case?  No. But it is difficult (impossible?) to determine if it will so to be conservative, it is assumed the doors are open.  This video was from arc flash testing by Jim Phillips for IEEE 1584 to determine the behavior of the arc based on the electrode orientation.