I don't know about salmonella regulations specifically, but most likely because A) something got past quality control or B) it's discovered that food safety standards aren't being met. A) When you're producing many units of something on a large scale, it can quickly get very difficult to check whether each unit is up to par, especially if testing requires units to be destroyed as is often the case with foodstuffs. There are organizations (the FDA in the United States, for example) that decide how much and what kind of testing is required. Because it's not reasonable to check every single piece of chicken, it's possible for defective/unsafe units to get through. On top of that, there are many different kinds of salmonella (some more harmful to humans than others) and it takes time for the infection to be noticed and then confirmed by a doctor. On top of that, it's hard to determine the source of the infection from just one case, and more testing (time) is required to figure out what kind of salmonella is being dealt with. You can figure out where an instance of salmonella came from before knowing exactly what you're dealing with. Even if you do know what kind of salmonella it is, it can still be worth recalling because B) It's possible the infections happened in the first place because corners were being cut during quality control or manufacturing. By not following the rules set by the standards organization, it can be difficult to tell what caused the problem in the first place and whether any other problems exist.
