Missing shear pin for tow-bar / front landing gear
The Capt would have just repeated or paraphrased whatever Maintenance Control told him.  If a push back is done with the nose wheel scissors connected, it is critical for the tug driver to respect the steering angle limits of the nosewheel to avoid bottoming the steering actuators and stressing the parts. 

Although the shear pin protects (hopefully) from major damage it still puts significant loads on the scissors and nosewheel steering system, beyond what those components would normally see.   If they find the towbar shear pin damaged or loose or sheared completely (although that should have been obvious while connected) after a push back, Maintenance will have to assume there was a steering angle exceedence event and there is likely to be a maintenance requirement to inspect the steering system, hence the return to gate.  

My guess is that when the tug got back to the gate, someone noticed the pin was deformed, or some other abnormality, and they brought the airplane back for the inspection to play it safe.