In this context, "what are you trying to pull?" also means, "what are you up to?". We use this often when we are suspicious of someone's actions. For example: "You have been doing sketchy stuff. What are you trying to pull?" "Why are you hiding? What are you trying to pull?"

"pull" as in 'trying to pull a rabbit out of a hat' trick or magic. Or it can be 'to pull your leg'. If someone is trying to pull a prank, they are trying to deceive you (not serious deception, but like to trick you) or to attempt something unattainable. If someone is pulling your leg, they are telling fantastical stories, they want you to believe.