The stuff underneath the risers always comes loose because of the heat in the (open) rig:-(
Doesn't anybody else have this problem? So all of my risers are 'naked but I'm really careful not to short anything. And this was not what caused my problem.
Please as much as possible do not change anything from riser you have bought, especially insulator under pcb riser, although your rig material made from wood this does not guarantee not short circiut. Better you keep using insulator, if no please you can looking for materials such as rubber or double tape to cover it.