I returned here after a bit of an absence to discover a big (for me) reputation subtraction for "serial up-voting". I've read some other posts explaining what this is supposed to be for, but none of what's described there seems to correspond to me. What is the subtraction for?
There have been a number of changes to the mechanics underneath the reputation system recently. You can find the details in the SO Meta Post Recent Reputation History Changes.
Serial up-voting and down-voting is the Stack Exchange version of stalking behavior. The algorithms that manage reputation irregularities now look for a series of votes in one direction to one account from one or a very small number of accounts in a short period of time.
Another post on the same issue shows an example:
That's 7 upvotes you received within 60 seconds. That's an average of 1 vote every 8.5741428 seconds...if all those votes were from a single user, then that's serial upvoting and those votes are not valid. The system was correcting that.enter link description here
It should work the other way as well - if someone down-votes a set of your posts in a short period of time those down-votes would be ignored and not impact your reputation.
A 25 point change isn't much and I looked and didn't see any particular day that corresponded, but one of the other mods may know better how to see that kind of activity.