MacBook HD "Other" all of a sudden almost full
Not sure whether you've already found this, but there are a number of other people having the same issue on the Apple Discussion Groups. There seems to be quite a lot of good information in that thread so I recommend reading it through, but the consensus seems to be that its hidden local backups/snapshots that's causing it.

A few people also used DaisyDisk to find, then delete, hidden space on the hard disk.