A hornet's nest is...a nest built by hornets. Hornets are ferocious, wasp-like stinging insects. If you disturb a hornet's nest, you cause thousands of hornets to swarm and attack. So, when you stir up something that is metaphorically a 'hornet's nest', you end up having to face a swarm of consequences.
We don't use "resto-bar" in the US, so I can't help you there.