Likely a result of battle damage, if you had taken any.
The GAU-8 goes though a clearing cycle within .5 seconds after every firing, designed to eliminate unfired rounds in the barrel.
There are generally a few ways for a gun unsafe light to trigger according to the -1 and 3-3's I've read:
Clearing Cycle fails to complete within 2.5 seconds after firing for any reason, including
Interrupting the clearing cycle by safing the gun
Two firing pins are detected as broken (unmodeled in DCS to my knowledge)
As a side note, the X's displaying instead of the proper round counter is either a product of further damage or improper modeling of a gun unsafe condition. Every example I've seen in documentation for unsafe gun displays still show an accurate round counter.
Kippy's post in So... What happened here was marked as the answer