Gum receded between two front teeth, could this be due to brushing or flossing? (Photo)

That area may heal.  Avoid flossing where you "saw floss".  Gently place floss under gum and wrap tooth and pull up only.  Repeat other side.  Level of bone will determine height of gum.Also possible to bond space or reduce teeth slightly at midline and move them together with minor ortho.

Hi:Normally, proper brushing and flossing maintains gum levels.  However if your gum is bleeding, most likely it means there is active bacteria growth.  It's best to get a cleaning.  Also make sure there is no clenching or grinding issue.Best,Dr. Maddahi

I would suggest using a WaterPik device on the lowest setting in place of dental floss.  Also, the area may be enhance-able using dermal filler injected into the papillae but I cannot see gingival grafting being effective long term.  Dermal filler would be the easiest and it can stimulate collagen growth over time.  You can mask the "dark triangle" by having the dentist bond composite filling material from the back sides of the teeth but it is not easy and replacement of the two veneers is another option.  I would try dermal filler first if it were me.

Unfortunately, it is very doubtful that this tissue will return. Grafting is a possibility but this is a very small defect so discuss your expectations with your periodontist prior to surgery. It may actually be easier to replace the veneers. 

Thank you for your question. The area between two veneers at the front, is quite critical for what are terms dark triangles. 
Your dentist would have warned you before treatment if it was likely that you would be at risk of having these. Some teeth are positioned in a way that makes it almost inevitable that this will occur. 
If your dentist planned to mininise the effect of this, ongoing hygiene maintenance (flossing regularly and seeing a Hygienist) would be the best ways to keep this fragile piece of gum tissue intact. 
Not flossing was not a good idea, and at the very front of your mouth, your technique would have to be very poor to be the cause of the recession.
As you rightly noticed, bleeding increased when you stopped flossing. 
Gum grafts are possible, very expensive, and for the minimal amount of gum to be increased unlikely to be a reliable procedure. 
It would be better at this stage to consider correcting one or both of the front veneers, to 'plump' the gingival tissue up slightly and make the visual appearance of the dark triangle less apparent. 
Redoing one would be cheaper and with a careful shaping, the asymmetry of doing it this way could be hidden.
Good luck with whatever you decide to do -
Thomas Dey, BDS, MSc