It would appear that this is the point where most force is concentrated, the knot loops 'clamping' down on the rope. Poorly tied or dressed knots will amplify this pressure by concentrating the force onto one small part of a bight of rope, rather than two even positioned strands. People who have done slow pull testing on knots state that the rope fails where it enters the knot because of the knot steadily binding down on the rope. Ropes used in rescue also fail where they pass over a sharp unprotected edge, not in the knot itself. This points to the need for letting the various systems and devices take the load for self-protection instead of your hands.
Water knots are an exception to the failure at the knot.
They fail by slipping under cyclic loading. Low loads, such as body weight, are sufficient to cause failure. Overhand safeties tied on top of a water knot may prevent the failure, but do not guarantee it. This is not all bad news for water knots. With an understanding of the mechanism of failure, you also know how to prevent it.