Just a guess, but I'd bet that the SIGHUP is sent as part of the System shut-down process.

Note: SIGKILL is not really stopable; you can catch a KILL and stick code in the subroutine to do cleanup (close files, delete semaphores, and such), but once your routine is finished, the System will continue shutting down your process. Kill is the equivilent of the 'Non-Maskable Interupt' that the hardware uses to inform you that "A Serious Error Has Occured, and I Am Leaving Now". (Often followed by an unsuccessful attempt to dump core.)