Orbits can intersect and naturally decay, at different rates according to different drag coefficients, that's why large pieces of debris require periodic avoidance manoeuvres as they cross others' paths, and the less predictable movements of debris from an actual collision would be a nightmare for years. Which means your space warfare strategy probably doesn't involve causing collisions unless you've contingency planned for losing your own assets.

I don't see any plausibly deniable scenario involving apparent spent rocket stages suddenly reanimating in militarily useful way. c.f. routine electronic warfare jamming. Even a "malfunctioning satellite" would have more deniability, and certainly equal ability to threaten others' space assets.