Another day, another user confused by one of these contextless messages. Losing rep is scary and without a good explanation these users end up both confused and asking yet another duplicate question on Meta.
How about we add a little "what's this?" in the form of a superscript question mark (?) or question mark icon linking to some proper description of what these mean? (or hovertext or make the words a link, something). The serial voting one in particular seems to often be read as implicating the user, which is never fun.
Additionally the "User was removed" and "serial Xvoting" descriptions are plain text, unlike normal rep changes are links. Most rep changes give a quick link to where the change happened, but these more unusual ones don't.