What causes a USB Cable to fry a PCB?
If you are shorting the VDD and VSS of the USB, you are not frying the keyboards at first. The Host PC is at the major risk of damaging the whole USB port. The Keyboard would be just an eye witness.    

Depends on the Model, make and luck anything of the below is possible:  


The USB port will get temporarily disabled
The USB port will let more than 100 mA or even 500 mA to let the cable or the connection fry including itself
USB will fail permanently to work   
The devices attached also would fry or fail.  


Have a protection feature to protect both your device of the interest and the host:  

https://www.littelfuse.com/technical-resources_old/application-designs/circuits/usb2-port.aspx