The most likely cause of this is either an old brew head gasket that is no longer forming enough of a seal or the small plastic ramps inside the brewhead are broken or missing.; they press the portafilter up against the brew head as you rotate it. It's also possible that your basket or portafilter is clogged and/or your coffee is ground to fine, but on a properly operating machine this would not result in the blowout you're experiencing. When you attach the portafilter, does it easily turn all the way to the right or is there some resistance? Try to brew with no coffee in the basket; do you still get the blowout effect or does the water dispense through the spouts as it should?