[roey2009]

Not necessarily correct. The reason of the invasion might be because of russian adherence to the heartland theory, creating defensive depth.

Might be Putin's megalomania, need for approval rating back home, maybe the oil prices rose and they took the opportunity, maybe to prevent Ukraine from joining the EU/NATO.

Could be a thousand and one differing things, or all of them at once.

What is correct? No one but Putin (and maybe not even him) knows.