What killed my process and why?

Try:

dmesg -T| grep -E -i -B100 'killed process'

Where -B100 signifies the number of lines before the kill happened.

Omit -T on Mac OS.

If the user or sysadmin did not kill the program the kernel may have. The kernel would only kill a process under exceptional circumstances such as extreme resource starvation (think mem+swap exhaustion).