After some chat-back-and-forth, I'm of the mind that this isn't Drupal or Drush related, but, rather, when you enabled the Devel module, your OS killed a process to manage a limited memory condition. Linux kernels will perform memory management by killing processes Adding a module will increase PHP memory, albeit temporarily, and your OS killed a process to manage the spike. I'm curious if you see whether there are "memory exhausted" entries in your site's logs — or, more likely, these will be occurring at the OS level. Stack memory exhaustion isn't too unusual on a low-RAM setup, or where your requested memory allocations, such as explicit Apache, PHP, MySQL or APC memory requests, use up too much memory, and the OS struggles to achieve a running state.
Try reducing your PHP memory allocation in conservative intervals, say, 16 MB, from 256 to 240 to 224, etc. Then test your changes while crawling your whole site to ensure nothing breaks. I'd do this on a staging server that replicates your live server, since the OS impacts how much memory your site will consume. For your site crawl, you can use a site crawler like XENU (Win only) with the Memory Profile module to see what page makes the highest memory request — and then add, say 48 MB on top of that, which should suffice. It is, however, an inexact process, since you must deal with a random number of random page requests.