I believe it is not the original reason: most UB are about funky HW. For instance for the non-wrap signed int it can be explained because C does not assume you have 2-complement hardware.
More specifically, some early hardware would trap on signed overflow. A lot of undefined behavior in C actually comes from "some machine would cause a trap", and C predates the invention of precise trapping in out-of-order processors. The possibility of traps is generally the difference between undefined behavior or unspecified/implementation-defined behavior.
