Unopened shredded cheese gets soggy in the fridge
Traditionally, mozzarella is  sold extremely fresh - as in made that day or the day before. If it's held for more than a day or so, it comes packaged in a brine. Most of the American stuff is just too soft to be held for packaging like other shredded cheese in the mega-mart.

In the US, being extreme gluttons for convenience, we tend to make do with part-skim shredded mozzarella instead of the good stuff. Slight temperature variations can make the shreds stick together in a globby mass. It's harmless from a safety point of view, and the expiry date can be long after the occurrence of perfectly safe globbiness.

Pro-Tip: Put the globby pieces in the freezer for 20 minutes, and then shred it like a block of harder cheese. Upon heating, it will melt (almost) as if the the globbiness had never happened. I have taken advantage of a few good sales that way. Mozzarella will become... ehem... colorful before it becomes unsafe. 

While this answer is more for mozzerella than for other types of cheese; the last paragraph works for any mega-mart shredded cheese.