[greesil]

Because he made a huge blunder.

Because he thinks this is how he stays in power. He set up this whole situation with a lot of optimism about Ukraine capitulating, or getting some concessions, and it didn't work out. Now he's attacking because he doesn't want to look weak by withdrawing the buildup. That might lead to him losing power, in his mind.

Or he's insane. The above is the only rational explanation I can think of.