My guess is that some systems delay writing to files for whatever reason, the problem being the avast.setup file isn't a permanent file, it is created to carry out an avast update and on completion of the update the file gets removed. So if there is a delayed write after the file is deleted, the delayed write would fail and the data lost as a result.
That shouldn't be a problem for avast as the avast.setup will get created when it is next required to Perform an auto update.
Though I don't know why you would have delayed write enabled.
