Your ELO ranking increases when you race well, and decreases a bit when you race poorly. The lower your ELO, the easier it is for you to increase it. Conversely, when it is high, it takes even more skill to keep improving. If you stop playing for a while, your ELO ranking will likely drop, as other players will have increased their performance since you last played.
Moreover, your ELO fluctuations will be impacted by the ELO of the players racing against you. If they are stronger than you, you are more likely to see your own ELO increase. On the contrary, if you race poorly against weaker opponents, your ELO will decrease sharply.