Given the size of the sparks and the fact that they’re gone on pavement, I’d say it is most likely just ESD. I’m 99% sure of it. It also likely wouldn’t be able to cause any significant damage to the motors given how insignificant the current would be compared to what the motors normally pull.
Insulators act kind of weird when it comes to ESD via friction. They can still carry a charge from it and then transfer it to other surfaces. A really simple method to see that is by rubbing a glass rod with a fur material and then bringing it near an object on a pivot point. It will either repeal or attract the object depending on the charge on the other object. It is a super simple physics lab done in intro courses if anyone wants to try it out or look up more information on it.
Here is one page I found with some more information for anyone who wants to read more. It should be easy for anyone to read and understand
