CPU's are digital circuits and the old technology was to built those
circuits in totem-pole configuration when one segment is on and the
other off. But on transition there is a short time when both segments
are conducting current and this is doing work (read, producing heat).
When you overclock, two possibilities, one the clock is too fast for the
matrix to respond and do something useful, two the whole package is
not build to release the additional heat produced, so bye,bye CPU!
Very roughly;^), dependant on linearity of the package to dissipate the
additional linearly produced heat. (Thermodynamics, enthropy,
refrigeration and other technical vermin)
Note: "Don't overclock unless you know what you are doing!"