I have heard that, we learned it in our Horse Health Management class at the UMN. I believe the worry is that it can cause purpura hemorrhagica which causes severe swelling and possible extensive skin loss (caused by pinpoint hemorrhages?) all over the head, belly, & limbs. I think its because the immune system is already sensitive the strangles bacterium and overreacts. This can be fatal. I'm pretty sure that you can get your horse's titer levels checked if you are worried about him catching strangles again. This will tell you if he still has immunity.
Equestriun - A vaccine works by stimulating the body's immune system with a weaker form of the virus or bacteria. It allows the body to know what it may have to fight against in the future. This way if the animal catches a virus that it has been vaccinated against the body already has a memory (there are actual memory cells that code for that particular disease) of what the disease is and can rid the body of it quicker. As far as I know the only way for a vaccine to cause a disease to become immune and stronger is if the body's immune system doesn't do its job and leaves some of the virus/bacteria alive which doesn't happen unless the virus/bacteria mutates quickly (like HIV) or kills the actuall immune cells (again HIV) as all virus/bacteria cells of a particular disease have th same proteins on its outer surface and this is what triggers the immune system attack.
Sorry for the book, too much info from school lately