New installer. Previously, Hazel was provided as a pref pane file that when double-clicked would launch System Preferences to install it. Unfortunately, Apple broke Gatekeeper in 10.11.4 where it (incorrectly) rejects any non-app software even when properly signed. Since the installer (being an app) is allowed by Gatekeeper, it will hand off the pref pane to System Preferences to continue the installation.
The “Bring to front” option in the Open action is now working again.