Serial downvoting is explained here and seems to be the downvoting of multiple questions / answers from a single user so that leads me to believe that Parallel downvoting would be the expression you're looking for: One user that downvotes all answers.

However I've not seem it happen much here on this site and I wouldn't worry about it as most high-rep users here have the sportsmanship badge which means they upvoted another answer to a question they answered as well 100 times...

Unfortunately, neither you nor me are in that list, but Zanna is whereas I'm only halfway there:

[![Sportsmanship progress][1]][1]

^{I thought I'd have more of those, but maybe first upvoting and then answering doesn't count as a sportsmanship vote...} [1]: https://i.sstatic.net/8xhzL.png

Serial downvoting is explained here and seems to be the downvoting of multiple questions / answers from a single user so that leads me to believe that Parallel downvoting would be the expression you're looking for: One user that downvotes all answers.

However I've not seem it happen much here on this site and I wouldn't worry about it as most high-rep users here have the sportsmanship badge which means they upvoted another answer to a question they answered as well 100 times...

Unfortunately, neither you nor me are in that list, but Zanna is whereas I'm only halfway there:

^{I thought I'd have more of those, but maybe first upvoting and then answering doesn't count as a sportsmanship vote...}