Q: Why iPhone bettery is swelling?

I bought a iPhone 5 in 2013 (Manufacturing date is May 2013 in Apple Box) and was using it for last couple of year. It was working fine so far and last week all on a sudden i see the upper portion of the iPhone is coming up, i was completely scared what is happening and why it is happening. Attaching a photo for your reference. Half of my top portion of phone is not attached to phone body anymore and it has come out. There is no falling of the phone, there is no water connect or anything external problem with this phone which can lead to this kind of issues. I went to Apple Service centre yesterday and the guy in the service centre told me the batter has swelled, hence it is coming out. I asked him why it is so, he was telling very rare case, but it happens some time. The charges he told to fix this is more than almost buying a new phone. All of us buy iPhone considering quality and customer experience at its best, but really shocked to find this with my iPhone? Apple Support team, do you have anything to say how this can happen without any external reason within 3.5 years of manufacturing? How much safety hazards this places, i wonder if you guys understand that if you don't feel that 3.5 years is long time and life of phone is over.