As mentioned in “How does your game fit the theme?”: multiple gameplay mechanics involve doing things in descending order, ie. counting down.
I’ll list some of them:
- gates need to be passed in descending order
- there is a special bonus for destroying rocks in descending order
- loose power ups are on a (invisible) countdown, the length is discoverable
- music/sfx use descending chord progression
- etc
And as you say the timer counts down.
If I had to summarise it, the game is a battle against the count down.