Not enough Oxygen in the root zone. I have these exact symptoms if My air pumps go off in my dwc buckets or the stones are blocked. erm in coco I'm not actually sure how to handle it. I have done a quick google search for you and I have found this it should help. Though this is my own personal opinion going from my 5 years of hydro experience. I could be wrong but I have run into this once before when one of my pumps broke and I didnt notice for well over a week mine was much worse than this though. I think it was one of my critical orange punch diaries you can see i put up a grow question too I thought it was an overfeed but I was wrong lol It could not be that its just as i said what has happened to me before.
https://coir.com/growing-medium/how-to-aerate-coco-coir-2/#:~:text=Replacing%20depleted%20oxygen%20can%20be,it%20to%20the%20plant's%20roots.