I have read this story in several places. The Russian satellite is described as being out of control. That can cause the orbit to be a bit erratic.
Unless it had thrust - it would be entirely ballistic and it's orbit entirely predictible
What I wonder is why we didn't see this coming at the old NORAD.
If in fact, the Sovs lost control over the satellite two years after they launched it - it would be in the data base but not necessarily 'tracked'
From what I can find this wasn't a head on collision, or a T-Bone, but more of a "side swipe". This will still cause a rain of space junk to settle through lower orbits until it can enter the atmosphere and burn up.
Bill G.