jackpotting
Holding back a bunch of stuff and then all of it comes at once, like a boss who saves all your mistakes and dumps them on you during a meeting.
He waited until the end of the year to fire me. He had all my mistakes ready to throw at me.
She waited until the end of the party to yell at him. She had all his mistakes ready to throw at him.
He waited until the end of the month to give her a warning. He had all her mistakes ready to throw at her.