There has been a trend for the last year or so where people have begun typing in all lowercase letters, and I've never heard an explanation. Ariana Grande is one of the biggest names that have taken to this trend on instagram.
From searching the web the two answers for this trend seem to be:
1. It's a cool aesthetic. It has both a uniform look (all letters are roughly the same size) and also a laid back feel, because it's not proper and punctuated.
2. Laziness. Apparently the trend originally started just out of sheer laziness. On informal forums online it just wasn't necessary to put in the effort of pressing shift of caps lock every time they wanted to capitalize something.
So ultimately it seems like this actually started years ago with the birth of the internet and people realizing that there's no need for formal capitalization. Then, in recent years people have adopted it as a cool, laid back aesthetic.