It says "voting corrected," but I hadn't noticed anything suspicious, and I'm pretty sure I haven't done any suspicious voting myself either. What gives?
Check your reputation change history. It's because of this:
At 13:53 server time yesterday, a user serial upvoted your posts a total of 19 times within the span of some 90 seconds. It is possible that whoever did this genuinely liked all of your posts, but regardless of motivation Stack Exchange does not allow targeted voting in either direction.
Accordingly the system periodically checks for and undo such behaviour. This occurred earlier today, and resulted in that -150 reversal you see. As long as you are not doing any of the serial voting (either up or down) yourself, there is nothing to be worried about. It wasn't due to anything you did.
For further reading see here for the official explanation, also linked to in the history.