Well it is a movement that could be construed as beginning his motion, it’s very obvious that the glove movement looks like that. So that’s the balk.
But I’d have to watch the entire series to give a thorough answer. Basically you can move your glove to signal new sign from the stretch before you come set. After you come set you cannot, any movement is a balk. It looks like he has come set based on his lean and positioning and the fact he isn’t taking signs, but it’s not obvious because his hands aren’t together. If he truly hasn’t come set and he has set precedent over the past few ABs then it is a sketchy balk.
If that’s the case I’d also note that pitchers don’t make it easy on themselves by getting way too creative with their routines.