General antonyms:
* Increase
* Escalate
* Worsen
* Intensify
* Flare up
* Erupt
* Rise
* Surge
More specific antonyms, depending on context:
* Emerge (if referring to something receding or disappearing)
* Resurface (if referring to something sinking or being submerged)
* Reignite (if referring to something calming down or dying out)
* Revive (if referring to something diminishing or fading)
* Aggravate (if referring to something easing or calming)
The best antonym for "subside" will depend on the specific context of the sentence.