I don't know the actual cause of the problem, but DRI is the direct rendering infrastructure, which is the code that allows X windows to talk to OpenGL (to make use of 3D hardware acceleration).
The crash would have been a fault either in libdri, or more likely in the particular 3D driver that is associated with your graphics card. The subsequent fault lines are just the cascade of errors after the first failure (the desktop failing to talk to X, then libdri trying to use a pointer from memory that was not initialized).
If it occurs repeatedly, consider moving to a more recent version of Mint (either wait for version 7 of the fluxbox variation, or use something like the xfce version).
If it still happens with version 7, it would be useful to submit a bug report (giving details of your hardware). You might want to do this through the Ubuntu bug tracking system
, and/or through the Mint one