There are several reasons that your program might receive a signal. You sent it a signal with kill, bkill, or bdel. The system sent it a signal because an error occurred or a system resource limit was reached. In this case, in addition to the exit code, the batch system will usually report an error message. The batch system sent it a signal because it exceeded a limit on the queue it was running in.
Most batch queues have a limit on the actual time that a job can run.