Well Gorby purged the military leadership of the "hardliners" (actual communists) before he started his reforms. He really thought the left would be a bigger threat than the radical liberal opposition.

He also drastically slashed the military budget and conditions for the soldiers were already poor in the late 80s. The conscription system had been corrupted and many conscripts were abused and used as slave labor. Cutting the budget made these problems worse, and then the Warsaw Pact collapsed and the army lost all their foreign allies.

So by the time stuff starts really falling apart, the Red Army is left disillusioned and directionless. They try to intervene a few times but accomplish little more than shooting some protesters and making everyone angrier. Gorbachev meanwhile refuses to lead, and instead focuses his attention on blaming his subordinates and firing more "hardliners". When the August Coup happens, again Gorby refuses to take leadership or even a stance on it at all, and what's left of the army isn't willing to risk a massacre on behalf of, let's be honest, a desperate last ditch attempt to regain control of the situation.

Afterwards, many soldiers could make chaos very lucrative for themselves by becoming gangsters or running drugs/arms. The death of a nation is so tragic.