It's actually normally a case of the opposite - unless you have a 3 or 4ft long pedal spanner it's impossible to rip the thread out unless you're Jeff Capes! Stripped pedal threads usually come about when the pedal is ridden loose. When aluminium parts are first fitted they undergo something called 'creep'. The top layer of the part in contact with another component (alloy crank arm, steel pedal axle in this case) hardens up, shrinks in size a tiny amount and allows the two parts to come loose from each other a little. Then, if you ride the bike like this, a small amount of movement develops and it's bye bye thread!