It might be because the partisans in eastern ukraine, the pro-russian partisans (so not the "little green men") are too crazy even for his taste.
If you look at the way things went down in Crimea, you had 2-3 days of chaos after which a seemingly sterile situation, tense, but clean, had developed. No big problems in matter of control.
You don't have that in eastern ukraine or SW ukraine in Odessa. You have citizen militias with ideological motivations and with guns killing people or getting killed. In Odessa, pro ukrainian partisans burned quite a lot of people in that building. In eastern ukraine again you have gun shots... and local strongmen calling for independence. Who are the "leaders" of those movements? They are russian partisans but they aren't subservient to Putin. They're like dogs who want to please their masters but instead of bringing the shoes nicely and clean, they chew the shoes and bring them full of slobber.
Putin likes things clean and taken care of swiftly. He has little tolerance for stupidity and the people who are entitling themselves as running the show in eastern ukraine are not very bright people. If you looked at the way they handle themselves in whatever press conferences they make and the way they talk...
In Crimea, there was a clear idea who was in charge of Crimea until he handed it over to Putin. Sergey Aksyonov. This asshole:
Sergey Aksyonov - Wikipedia, the free encyclopedia
And you can bet that he wouldn't say a word without Putin telling him what to say word for word. He got his orders and he did exactly what he was told to do.
In eastern Ukraine you have a dozen self-entitled mayors or governors or whatever of all sort of cities calling for all sort of crazy demands which clearly Putin didn't tell them to demand. They asked Putin to enter with his army in the very first days of that Donestsk Peoples' Republic, remember that? That new "republic" that was 1 building that they fortified and right after declaring independence, they asked Putin to send in the army. It's idiotic at best. Ofc you want to distance yourself from such people.
Secondly. If Putin is to do something, like send in an occupation army with no brand like he did in Crimea, he wants to be perceived first as being the guy who decided to ease the tensions. So ofc he's gonna pull back.
SHORT VERSION:
1) Because the "leaders" of the pro-russian militia are disorganized loons who yell and aren't disciplined and possibly either too stupid to follow instructions from Kremlin or too enthusiastic and you don't trust or do business with undisciplined idiots as a world leader.
2) Because if Putin is going to pull another Crimea with a "stealth' invasion, he wants first and foremost to seem like he isn't doing that.
This is another good explanation for why he pulls back. Also, the 1) and 2) I mentioned above are also, in my opinion, legitimate explanations for this.
Anyway, it isn't NATO.
It could be that the sanctions are starting to bite harder than he thought and is trying to make a peace now with the EU and the US to avoid the next round of sactions. But you know, judging from everything we've seen thus far, I would rank this as the least likely of explanations.
