The best explanation I've been given on this is by a defence analyst in Moscow I spoke to about a week before the invasion, who explained to me that the Russian authorities had come to a decision that the Ukrainian state was inherently hostile, that it had been for 30 years, that it was incapable of making peace over Donbass and that it was being egged on by the west; therefore, there was going to be a war. If it wasn't today, it would be tomorrow or it would be a year from now or in five years from now.
They were in a position, you might say, like the Germans in 1914, who were convinced that war was coming and it's better to do it now while you still have a chance, rather than five years down the road when there are NATO troops there and you start World War III.
That's the most logical explanation I've been given for Putin's decision-making in this context. In his eyes, therefore, it's sort of preventive war because they'd convinced themselves that this was going to happen sooner or later.