To extend on this, I know some people who actually speak louder precisely when they do this. It's hard for me to say whether its intentional on their part, but it certainly feels that way - and a bit malicious too. Kind of like "What I have to say is more important than what you were going to say". That aggravates me because at that point it stops feeling like an accidental cut off which like you mentioned, can happen naturally on occasion.

These days when people start doing this, I just completely stop talking and no longer further contribute to the conversation.

Of course, when I try to carefully bring this up to said people (just in case it isn't intended) I'm made out to be the bad guy which I really don't understand.