(THIS ACHIEVEMENT WILL UNLOCK FROM NATURAL PLAY IF PLANNING TO 100% THE GAME - NO NEED TO WORRY ABOUT IT)
Throwdowns are located in the career section under campaign. Press
under campaign to be able to see them. There are a total of 13 throwdowns, the unlock requirements aren't exactly known, but most likely tied to events completed and total number of stamps.
The last throwdown I unlocked was "GATE CRASHER CLASH" after completing the event "RALLY RAID: THE REDEEMER" with 295 stamps and 103 events completed, I completed every event before this from top/down. I say the requirements aren't exactly known because other players have reported to get this achievement much sooner with less stamps and events, so I'm assuming that events completed and number of stamps aren't the only requirements in unlocking throwdowns. The last throwdown I unlocked was also different from the last one they unlocked.
If you know specific requirements in unlocking a throwdown, comment below and I'll add it to the guide.