It was an urgent situation, not something that was pre-planned.
The people in Crimea got seriously worried about a Majdan type situation there, and hooligans from that neo-nazi organisation coming there and wreaking havoc (which they had said they would).
So various leading figures locally decided to take pre-emptive action by blocking action and talking about a referendum. Russia was more than happy to play along, with green men and everything else that followed.
There wasn't time to stop and think about water viaducts and gas pipelines or whatever else has been causing problems after Ukraine started blocking.
Plus, I think that as things progressed, the Crimeans got re-assurances from Moscow that Russia would have their back whatever happened.
I think Russia was prepared to do pretty much anything at all to help people there and was absolutely over the moon about having Crimea back.
It somewhat unfair though - that Crimea is treated like the Prodigal Son whereas Donetsk/Lugansk are on their own, and really suffering despite turning to Moscow.
It's a complex situation but Russia decided to pull out all the stops for Crimea but not for Donetsk/Lugansk.
On the Transnistria issue: Thanks for the update Basil77. That is SO sad to hear. Of course, this is not covered in Western media at all. Nobody knows anything about it. I know I said that the country is "cool" and that's true, but there is also a lot of poverty and problems there, so in a way, my comment was stupid.
They are cut off from the international monetary system. A lot of infrastructure was in a really bad state - for example I travelled in a tram that looked like 1950s vintage - it was fixed up with duct tape to keep it together.
People there are forced to live on really low sums of money - I'd guess a few hundred dollars a month if they are lucky. All sorts of shady business going on for people to stay afloat. That is not "cool", it's sad. It's the spirit of the people that was admirable. I spoke with a woman who had moved there from Moscow to be with her husband who was local She said they had chosen to live there, rather than Moscow because the country needed them. Admirable, since this couple was well-educated and could have a much better living standard in Moscow, no doubt. There were lots of banners and slogans saying things like "Friendship with Russia forever" and similar. Plus, instead of removing Soviet symbols, I got the impression they actually put up more. And everybody seemed to be religious! It was a very unique kind of place with a very resilient and gutsy population.
I wonder if Donetsk / Lugansk will end up in a similar situation as Transnistria and Abkhazia - in limbo / vacuum.