The term you're looking for is debundling. Debundling is definitely not just a UX decision. It's a collective decision between business technology and design. There are many reasons why not to debundle a software and same amount of reasons why the product should have been debundled immediately.
First major debundling seen from Foursquare. They've released a fun app where you can tag your location, leave reviews etc etc. But then they've received a huge interest and then they've started to collect massive amount of data which made them a good rival against yelp. But the thing is, foursquare released as an startup and probably didn't had the structural code hierarchy of an enterprise company (just assuming here) Instead just piling the data they were looking forward to monitise the data so as a business decision they've decided shift the company model into a data company instead of just an entertainment company. But also they didn't want to ditch their user base. Doing the two different complete business under one brand would be just a chaotic mess as seen in FB atm. So -as a very short- they've come up with a another app called Swarm for the users and kept the entertaining part in there and reshaped Foursquare into a way more enterprise company. Well I'm not saying that it's been executed pretty well but they're still a strong name in their field and seems like everything been worked out just fine for them.
Another example would be Google. They have dozens of apps which they use the same password but doing completely different things. It would be HELL if I only had a 'Google' app where I can check my mails, download files from drive, watch videos on youtube, start a video conference, etc.
I don't know the technical details but they do have a shared API that talks between google apps within iPhone. So logging in within a fresh installed Google app on my iPhone is not a problem. But their decision to debundling drive into different apps I find a bit annoying. I could literally do the same edits within drive and if I happen to have docs or sheets installed by mistake, I end up jumping between apps and can't understand what's been going on. If I'm using drive on iOS, I'm always lost (that's why I don't prefer using it on mobile devices but from desktop)
Last example would be Facebook. Basically they HAD TO debundle their application because they were literally living the hellish Google app example I've mentioned a paragraph before. I remember at some point their application was so bloated that it was over 700mbs. Most of the iOS games takes that much of a space if they have 3D graphics and high res textures. Those are the ones takes up space. Facebook doesn't have that. Just lines of codes. So at some point it became harder to release a stable app so they had so debundle it into several different apps. (they all failed so they've came to an understanding that they didn't needed to have 4 different snapchat clones and another few clones of their own messenger apps)
So my point is, line could be several different reasons. Could be business related, technology related. Oh yeah, sometimes it can be even UX related too. But mostly business and technology.