I’m surprised that what seems to me to be the most obvious hypothesis isn’t covered. Until very recently, historically speaking, written and spoken language have been very clearly separated forms of language use.
With the advent of the internet, instant messaging, social media etc, the distinction has been gradually blurring in the sense that written language is being used more and more frequently for what would previously have been considered ‘spoken uses’.
We know spoken language mostly consists of shorter fragmented phrases compared to the longer complete sentences of the written word. It should not be surprising that as writing is increasingly used for ‘written speech’, regular writing will be influenced to move in the direction of the shorter phrasing of spoken language while otherwise maintaining the syntax and grammar rules of written language forms.