For starters, I don't use my MacBook Pro very often. I usually left it conveniently on my bay window indent so if I was sitting in the recliner, I could grab it and watch TV, etc. One night when I fell asleep on the recliner, I woke up at 2AM to the most dastardly loud "pop" that I had ever heard. I had absolutely no idea what it was, looked outside, moved furniture, even looked at the windows because I thought maybe one of them got cracked. Fast forward one week, I go to use my MacBook and notice it's dead. "I must have let it sleep with low battery", I thought, as I went to plug it in and realize that the trackpad is popped out from it's position on the verge of being torn from the MacBook. Having no idea how this magically happens, I Google it and, low and behold, I find an Apple Forum where someone has the same issue and their battery exploded. After unscrewing the MacBook, it turns out that the battery finally blew up, which was pretty upsetting because I had always made sure to perform Apple's battery calibrations every few months to prevent things like this.
The repair went very smoothly. The guide on iFixit is on-point and I was able to take the power connector off, unscrew the three screws with a properly-sized flat-head screwdriver (since they're actually 3-sided screws), rip and replace, and the battery was good to go. The best part is that the trackpad settled back into its place as well!
This could have been numerous things, but I think the actual cause what that it was 10ºF outside and my MacBook was by the bay window, so the cold may have pressurized the battery/computer and caused it to pop. Keep your MacBook in a location that may not be affected by the weather :)