What would make sense now are only two scenarios:

1) MTGOX had encrypted the keys and couldn't decrypt them any more - now checked all possible decryption scenarios until one worked. Now - to avoid having an "all in" scenario again - they decided to split them and handle each wallet with a different security.

2) The Bitcoin transaction in 2011 was a transaction by someone close to MTGox did them a favor to proclaim liquidity. When he now found out that his wallet is under suspicion, it gave him the creeps and he moved them to several others.

3) The "thief" theory is - reasons already mentioned above - off the table.

well yeah, and the big time fraud theory by someone from MTGox, believing authorities will be unable to ever catch him - i hope at least not.