Negotiating leverage, and I'm not sure this really is a "massive buildup", it is a buildup, but as you note he's had tens of thousands of troops stationed near Ukraine for years. Perhaps he wanted to get them noticed by moving 20,000 or so to a forward position. They have to be somewhere, so the expense is in building quarters for them nearer Ukraine. So, it's a gambit, and also provides him with a credible threat to invade, which probably is in the back of his mind.
He could well claim ethnic Russians in Donbass are being persecuted and move into that region.