Why running cron tasks would hit a REST service endpoint?
It's impossible to answer the question directly because we'd have to speculate about the all the possible states of your Drupal installation where rest_server_server() is hit in a a cron context.

However, since you know where the notification message is being produced, and you can trigger it in a Drush command, what you can do is insert something like debug_print_backtrace() in the rest_server_server() function then re-run the Drush command to get an idea of what module/configuration is causing that to occur and if it's causing cron to die on you.