since the bobsled is moving on a path that is the shape of a tube around curves, the mass and velocity of the sled creates centripetal force, which is the force that pulls the object towards the center of the circle, in this case it is what is preventing the bobsled from moving too low or high on the track (keeping it in the middle. this way the bobsled can have the perfect speed.
acceleration due to gravity, which is 9.81 m/s^2 is a key factor in centripetal force.
friction also plays a role. the metal skate od the ice has very little friction, but it doesn't need a lot to throw it off. so the bobsleds racing later have more friction to overcome due to the previous teams marking up the ice.
momentum, mass and velocity, play a huge role in this as well. the more momentum they have the easier they can overcome the friction and the drag that would slow them down and cause them to not be in the center of the tube.