FranticFish - 12:19 pm on Apr 26, 2013 (gmt 0)
There are two reasons this could have happened:
1) You did harm rather than good
Perhaps the new combination on-page repetition of the k/w and of inbound anchor text (assuming you have some) was considered not as good as it was (you drop) or even abusive (you get a penalty).
2) Google's 'confuse SEOs/tweakers' algorithm kicked in (if it exists)
This is patented but not confirmed as being active (at least I've not read or seen pronouncements from Google). In which case, sitting tight should see things return to at least as good a position as they were.
Personally I'd sit tight for a few weeks. If you've kept an exact record of what you've done you could roll back once you're SURE you're not coming back. That might not get you back to where you were before though.
In the meantime, undertake a very thorough check on your IBLs (and if any have anchor text links, how those are spread and how they relate to the k/w in question).