Not forced into a corner exactly, but there are a lot of people who regard themselves as Russian living there and who he is expected to defend.
Why is Putin expected to defend people living in Ukraine? Isn't that the job of Ukrainians? There are Mexican nationals living in the US as citizens, is it the job of the Mexican government to protect and defend those people or is that the job of the US government?
Their government, whatever you think of it, was overthrown and they wanted to go back to Russian rule.
The solution for that problem is to move to Russia. That's where Russian rule is. If Ukraine is under Russian rule, then that's an obvious problem in itself.