ffmpeg closes with return code 137
137 (128 + 9) means it's been killed with signal 9 (SIGKILL). This usually means that something sent ffmpeg this signal, not that the program crashed on its own.
Could it be that there is some process watching for long-running ffmpeg and killing them?
Another possibility is that your ffmpeg process oversteps it's ulimit quotas and it gets killed by the system (although I'm not sure if it would get killed with SIGKILL and not SIGABRT in that case). Run the command ulimit -a and check for suspiciously low quotas.