@veyron_fan1First, every drivers in the game (except icons) had a growth and decline cycle through the entire 10 years seasons. Their base stats and ratings will naturally goes up and down with set values regardless of anything happening in your career.
Activities are straight forward boosts to their stat that last only one race, while your facilities' boost are also a direct boost but last forever as long as they remained in your team.
F2 drivers in general had a steeper curve on their growth cycle than F1 drivers, but because their base stats are so low it didn't matter much, at the peak only a small numbers of them actually reached the level of a midfield F1 driver.