Etwas hochkochen lassen — to let something boil up. Said of a person who allowed a conflict to escalate, and it is an accusation.
Das schaukelt sich hoch — it rocks itself up, a conflict building through mutual response.
Öl ins Feuer gießen — to pour oil on the fire.
Die Wogen glätten — to smooth the waves, the approving description of what a good manager does.