Drush make Distribution throwing errors: exec() unable to fork bootstrap.inc:636
This is probably related to some resources limit or temporary glitch. So first try to restart your Apache.

If this won't help, make sure that you've necessary requirements to run drush (which is either curl or wget).

Otherwise it could be some resource limitation on your environment. E.g. check for ulimit -a or kern.maxprocs.

If there is still problem, please add -v for verbose mode or -d for debug messages for drush, you could more easily track the issue.