You'll never know what caused it from being a "glue issue"... The only way would be if the substrate{paint}failed,wasn't adhered to the car and left loose leaving it stuck to the vinyl top but that's not the case here from what you've stated..
Other than that it could be old glue,too long between glue application and assembly, not enough glue,temperature issues etc.. I've run into a situation like this before and it was the bottom of a 5 gallon pail the upholstery shop was using..