A few common reasons for this:
Portafilter is not being locked on tight enough
Basket is being overfilled
Gasket seal around the shower head isnt clean
When I first picked up the silvia I was locking the head until it felt firm. It was only when further tightening it that it stopped leaking ie I would tighten it until the nossels on the bottom of the portafilter were parellel to the machine.