When I logged on this AM, I found that I had been deduced 12 reputation points because "Serial Voting was Reversed". Would someone please explain what I did that triggered the deduction, so that I can avoid making the same mistake again?
You most likely didn't do anything to trigger it directly, though you have led to it indirectly.
By raising a flag earlier to notify moderators about a case of serial downvoting, I ended up escalating the issue to try and have reputations restored to normality, which itself led to a chain of events that eventually culminated in a point recount made by the system.
From then on what the system does is really a black box to us, it may lead to favorable or unfavorable results to an undetermined number of users.
If you report a rep loss, then I presume among other things the system considered you were perhaps being favored by some other user, which may intentionally or unintentionally have upvoted an abnormal amount of your posts.