Well, I think it's one (or more) logfiles which have grown too large and need to be removed/backupped. My suggestion is to go for the big files first. To find all files greater than 10 MB (10 MB is a big enough file size, you can choose +1M for 1MB similarly)
sudo find / -type f -size +10M -exec ls -lh {} \;
and now you can identify which ones are causing the trouble and deal with them accordingly.
As for your original du -a / | sort -n -r | head -n 10 command, that won't work since it is sorting by size, and so, all ancestor directories of the large file will go up the pyramid, while the individual files will most probably be missed.
It should be pretty simple to notice the occurrence of similar other log files/binaries in the location of the files you so find, so as a suggestion, do cd into the directory containing the original file to clean up more files of the same kind. You can also iterate with the command for files with sizes greater than 1MB next, and so on.
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