Talk:Generations/@comment-27759872-20160406123704/@comment-27192151-20160408052949

Yes, there are three ways to unlock a bonus series:

1) Collect enough trophies in the respective career series (which is probebly why it was unlocked in your case)

2) Purchase a car used in the series using get it now

3) Completing an event that has one of the cars in the series as a prize (this might not always be the case though, as I believe MotorFiesta 1 doesn't unlock a bonus series, at least anymore)