It was not the battery, the motor is specced for 3-4s. Capacity/c-rating makes no difference--you cannot burn a motor out from a battery with too much capacity; current is 'pulled' from the battery, not pushed
On the back side of your motors, there is usually a little bit of the smaller shaft protruding out, such that it will rub on the plywood mount if there is not a centered-cutout in the mount behind it. Does your mount have this? i.e. is the motor shaft on the back side rubbing on the plywood piece? My thought is that you overloaded the motors with extra load somehow.
Still, it is extremely weird to me that they both blew at the exact same time. The prop and battery you used are sized just fine for those motors. If it was a wiring issue with the battery into the ESCs (crossing lines or something like that from lack of heat shrink somewhere, idk), the ESCs would've blown first.