The rear spark plug boot, the kind that goes down into the center of the engine, pops up about a quarter inch every so often. I have switched the boots and cleaned the walls of the plug area and it pops up on the rear cylinder only. This is a 2002 Passat 4 cylinder Turbo with 163,000 miles on it.
Possible causes could be:
Spark plug loose or plug gasket not sealing and this is allowing cylinder pressure to leak out and blow the coil out.
Aftermarket plug boot that does not fit quite right.
Original or aftermarket boot that is aged, oil soaked to some degree, and the rubber has swelled and softened. This leads to the coil/boot trying to push itself back out.
It’s a cry for help. Maybe replacing the spark plug wires will fix this. These wires don’t last forever. They wear out and have to be replaced every now and then.