What is somebody playing at?
Definition 1 of 1
Pro Tip 1/3Suspicious Context
Use it when you doubt someone's actions and need an explanation for their unexpected behavior. 
He's been very secretive with his phone lately. What is he playing at?
Pro Tip 2/3Informality
It's casual and might be rude in formal or serious conversations. Use with friends or in laid-back settings. 
She's taking another break? What's she playing at, we've got work to do.
Pro Tip 3/3Colloquial Nuance
This phrase often carries a tone of frustration or annoyance with the other person's behavior. 
They promised to help us but now they're backing out. What are they playing at?