"Human error" is blamed. It covers singleplayer. It covers multiplayer. It covers Trueskill leaderboards. It covers all matchmaking data. Gone. Says the Major:
This was human error related to Xbox LIVE servers. It is not a broader service issue and will not impact other titles or Xbox LIVE experiences. The Xbox LIVE team is investigating the cause to prevent this from occurring in the future.
Larry goes on to apologize, on behalf of Xbox Live ...
"for the incident and any frustration Halo Wars players have experienced in having their statistics disappear from the Leaderboards. You can rest assured that we're putting processes in place to prevent this from happening in the future.
Halo Wars Leaderboards [Major Nelson, thanks Brian B.]