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The reason behind the usage of pre and post-increment resulting in undefined behavior in these constructs is because of their side effects. In C++, the order of evaluation of function arguments is not specified, and the side effects of an expression can occur at any time during the evaluation of the expression. This means that the behavior of code that depends on the order of side effects can be undefined.

In the case of pre and post-increment, the behavior is dependent on the order of the side effect of incrementing the value of a variable and the order of evaluation of the expression that uses the value of the variable. If the order of these side effects is not specified, the behavior of the code can be undefined.