These "bends" (or sloped straight lines) are due to various structural constrains on the aircraft.
In level flight, the sum of forces must be zero:
LiftWing - Weight - LiftTail = 0
The summation of moment should also be zero, otherwise the plane will pitch up or down.
The wing has a structural limitation, i.e. a maximum loading. Solving the above equations for constant LiftWing yields a slopped straight line. This is the "bend" portion you see in the graph.
Drawing these lines (shown as colored lines here) on top of a weight and balance graph yields:
The resulted graph is the intersection of the various constraints.
In the C-182 graph, there are five constraints (five edges). Wing loading and tail loading is used as an example here, since I do not know what is the actual constraint used by the engineers. Another possibility is strength of the nose gear.
Main landing gear loading and aft tipping limit would have resulted in a downward slope, which does not correspond to the given graph.
Source: Center of Gravity Limitations (PDF)