I would say mechanical forces, but a common way they are increased is by raising RPM, as in Ruins Peoples' Motors. The first step many take is to remove the governor for $0. Then that's not enough, so they might get some $12 valve springs. Then they're spending $120 on a new Predator because RPM ruined their motor.
There are a lot of reciprocating parts in a piston engine that have to change direction very quickly. The acceleration factor increases to the square of velocity, so going from 4000rpm to 8000rpm (a velocity factor of 2) means acceleration goes up 2² = 4 times. Acceleration doesn't happen without force and force = mass × acceleration, so when acceleration goes up 400%, so does force. When that force is higher than parts are capable of, the parts exit crankcase left.