All Bitcoin mining clients have to be bug-for-bug compatible with every other client or else you get forks like these. I think the fork in question was caused by a miner running a recently released version (v8.0) of the software who mined a large block that was incompatible with the older versions (<0.7x) of the software.
The miners running the old software didn't recognize the block and instead created their own chain while miners with the new version were working off of that newly mined block.
The miners running the old software didn't recognize the block and instead created their own chain while miners with the new version were working off of that newly mined block.
https://bitcoin.org/en/alert/2013-03-11-chain-fork