Split/Second

Bonus rounds unlock by completing milestones that the season wants you to do, like wreck 25 opponents. This achievement requires you to only win six but there is another achievement to complete all the bonus rounds. Most of them are detonators which are hard because they want you to beat a time that is almost impossible without doing everything perfect. Here is the list of bonus rounds based on season name, map/event, and unlock requirement:

Rigged to Blow
Airport Terminal - Detonator
20 Wrecks in the Season

Splashdown
Air Strike - Port Bridge
25 Wrecks in the Season

Big Rig Boom
Air Strike - Downtown Central
20 Wrecks in the Season

Sunset Fire
Dry Docks - Detonator
30 Wrecks in the Season

Hit the Skids
Expressway - Detonator
20 Wrecks in the Season

Midway Pass
Power Plant - Detonator
20 Wrecks in the Season

Threshold Break
Airport Terminal - Elimination
25 Wrecks in the Season

Cleaning Up
Storm Drain - Detonator
30 Wrecks in the Season

Scorched Rubber
Ferry Wharf - Detonator
25 Wrecks in the Season

Boneyard Rush
Canyon - Elimination
30 Wrecks in the Season

One Last Push
Desert Run - Survival
30 Wrecks in the Season

Out With a Bang
Airplane Graveyard - Air Revenge
25 Wrecks in the Season