So I have dealt with a lot of injuries in my day and have never had a horse get cellulitis.  We moved about a year and a half ago and since then I have had 2 different horses get it from very minor scratches that I typically wouldn't even treat.  It is possible the ground is contaminated with something?  Its the only thing I can really think of but wondering if anyone else has ever dealt with this.  

My mare gets it every time she gets cut or scraped on the paddcock fence on her back legs. She rolled into the fence one day and just skinned the inside of her bacl leg halfway between the hock and stifle. It was barely anything, I just left it alone since it wasn't bleeding & she was fine. Next day her leg was huge and probably just a few hours away from the skin oozing. She's into the fence almost daily & it's never bothered her before. But who knows, it could have been something on that specific board. No other horses have had issues with it. But it is a recurring thing, anytime she cuts that leg on anything now it starts to get big within 2 hours and I know to just go ahead and do a compression wrap & antibiotics on her. I have no idea in your case, but if they scraped themselves on or near the same object, its very possible. My vet said it can also vary by horse as well, because several others have gotten into that fencline and they never had a problem 

Bump

I had a gelding get it once. Seemed to come out of nowhere. I did take to the vet and they said it can be dangerous if not treated aggressively. Back leg swelled up like a balloon and he was very lame. 

I would get her on a immune booster.  There are several good ones.  Animal. element has a really good one.  The immune system may be compromised and when she gets a cut or wound her body is not able to fight off the infection.