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All two-dimensional hard regular polygons have stable crystalline structures above a finite phase-transition density, and adding short-range interactions is believed not to qualitatively change their thermal stabilities. By molecular dynamics simulation, however, we find that adding a short-range Lennard-Jones interaction on each vertex of the hard regular triangle leads to ground-state degeneracy of the triangular system due to the fact that the unit cell of the crystal formed by a dimer has two isotopic structures with identical potential energies, which induces spontaneous crystalline instability and allows the system to be in the glassy state at any finite temperatures before liquidizing. At a fixed finite temperature, the initial crystalline structure eventually evolves to the glassy state spontaneously through a down-hill process after it comes over a single energy barrier. This mechanism is further verified by the observation that forbidding one of the two isotopic structures of the unit cell extremely heightens the energy barrier.