http://sparrowa7io5pz6ud3ehqzosvepbxbxt2zphmkjsylp2zgxooko23pqd.onion/docs/faq.html
If Sparrow cannot connect via the configured proxy, but the server is using an onion address, it will start it’s internal Tor proxy automatically.
You can disable this behaviour by closing Sparrow and changing the value of the autoSwitchProxy property in the config file.