One fairly plausible explanation is that Putin is not, as everyone assumed, an evil genius, but is rather an evil idiot. But it was probably largely to shore up his internal position (one of the downsides of being a dictator is that your retirement options are limited, so they often go very odd in their old age as they try to avoid appearing weak), and that might even work. There's some indication he's more popular internally than he was a couple of months ago.
Like, there's no outcomes here that are good for Russia, but there are outcomes which are good for Putin personally, and that's likely all he's interested in.