Time for some personal speculation.. May be far off of course, input welcome.
I think Crimea has been the target of Russia since the end of the Soviet union, and they have just waited patiently for the time to strike.
I used to play a russian developed flight sim back in the day, SU-27 from 96 I believe. It was set in Crimea, and Crimea was the only map. I mean, it was pretty clear to me that even back then, they were practicing and researching and simulating different scenarios for how to take it back. The speed and smoothness in which they have now taken it, supports this theory.
To me, it seems they have just taken a fitting plan from the drawer now that Ukraine is in a weak position. The way that Ukraine has become weak is irrelevant for their plans.
It can even be argued that Crimea "belongs" to Russia more than Ukraine, and I wouldn't disagree completely.. Still, letting Russia just annex part of another civilized country is not something that can be accepted by the world.