Note that I do not know for sure that it is bubble disease in this case - no one can be that. However - bubble disease is an example of a disease that has an abiotic origin (abiotic means - non living). Together with the well known connections between environmental factors (abiotic factor), stress and outbreak of diseases of biotic orgin (living faculative pathogens and also viruses. see @Brew12´s very excellent article about this or this article ) abiotic factors may be responsible for a very high percent of disease outbreaks in fish.
We tend to strive for explanations that we cannot be responsible for ourselves - we always concentrate in something that comes from outside our control as the cause of a disease outbreak.
I´m sorry for your lost but glad for the ones that survive for you.
Sorry, I just saw this but would not have been much help from that one picture. I stir up my gravel all the time but never experienced this problem although what Lasse said is interesting. I have never heard of it which is why I would not rule it out. I just can't tell from here and would be guessing.