Yeah, it's beause the yeast died. It probably would have been fine, you probaby didn't store it in a "cool dry place"
Most things are so loaded with preservatives it's amazing that they go bad at all.
I only look at the expiration dates for amusement. You can tell when something is bad...
Most stores give all of their expired goods to the food banks. There is nothing wrong with it, they just can't guarentee that its fresh.
Even milk is good for 2-3 weeks passed the expiration date if it is kept near freezing.