Looks like both you and Jaida have been through a lot! And it seems like you are coming through it well.
What is a seroma?
It is an accumulation of serous fluid. What is serous fluid? It is blood without most of the cells. When you add the cells back in, then it is called a hematoma.
Where does it come from?
Inflammation is the typical cause. In this situation? The patient is a strong muscular dog - any type of contraction of the muscles that remain (you wouldn't really be able to see this well from the outside) can lead to leakage of fluid. Reaction to the suture can lead to inflammation (some dogs just react to suture more than others - I have a client rechecking tomorrow for this reason; it isn't common but does happen). Trauma can do it - If Jaida layed down too hard on the carpet for example. Something simple. Then once some fluid starts, it can lead to some additional inflammation and then the fluid will get larger. I noticed on the previous thread there was a common as to why the fluid was on the sternum when the incision site was higher up. The answer to that is gravity and osmosis will draw the fluid ventrally to the sternum. That is why drains are placed low on the patient - they drain better.
There are other causes which would be unlikely - these may include platelet problems either in number or ability to work; blood protein issues, liver function issues, immune mediated, etc... It other patients where cancer was the reason for the surgery - that can be a cause as well.
You asked if the pressure bandage could have caused it. Unlikley. It is possible that rubbing of the bandage next to the general incision area may have led to secondary inflammation. Pressure bandages typically reduce seroma formation, not create it.
Regardless of the cause, the important thing is that Jaida sounds like she is doing better. If clinical signs re-develop, then I would re-evaluate with your veterinarian. Sounds like he did a great job. Sorry to hear that you had complications. No matter how 'perfect' the surgeries go, complications seem to arise on a random basis.
I hope this answered your question and was helpful. Let me know if there is anything else I can do.