Why now?

Likely the world oil price and nothing more. Putin depends on a large patronage network — if he can’t pay his thugs, and pay them MORE than anyone else around who could replace him, he’s not going to retain his power. And be dead very quick. Just ask Capone’s rivals how that works.

Putin is sort of a giant-sized Capone.

Also, the West and specifically Bush is weak, and weakness invites attack. Putin probably thinks Obama is a done deal, nothing could stop that from happening. But the biggest thing is getting Oil back above $145 a barrel so he can meet his paydays.