Why does my battery keep expanding?

Hello, I bought a replacement battery for my Galaxy S7 Edge in March and everything was working fine until around July it ended up expanding slightly which opened up the back of the phone. I got it replaced under warranty by the support here. However, I am now experiencing the same problem I had in July. The back cover has opened slightly which leads me to believe that the battery has expanded and the phone runs out of charge randomly. I did the replacements myself but I had no problem doing it. Have I just happened to receive 2 defective batteries or could it be some kind of user error? If it is the batteries, can I get 2 replacements under the 1 year warranty or do I have a buy another one?

Thank you,

Luis