0.8 and 0.7 have different databases (LevelDB and BDB respectively). The 0.7 database doesn't accept some high-volume blocks under certain conditions (not 100% sure what those conditions are, that's for the devs to fix now). As a result, the chains forked when an 0.8 node mined a block 0.7 could not handle.

Everyone followed proper behaviors, it's just a compatibility problem. So miners should resort to 0.7 until 0.8 can prevent a fork like this from happening again.