The backup docs http://source.wiredtiger.com/2.9.3/backup.html say:

The cursor must not be closed until all of the files have been copied, however, there is no requirement the files be copied in any order or in any relationship to the WT_CURSOR::next calls, only that all files have been copied before the cursor is closed. For example, applications might aggregate the file names from the cursor and then list the file names as arguments to a file archiver such as the system tar utility.

I thought WiredTiger required applications copy the files in the order the cursor lists them, for example, log files first. From __backup_start():

 * Include log files if doing a full backup, and copy them before
 * copying data files to avoid rolling the metadata forward across
 * a checkpoint that completes during the backup.
 */
target_list = false;
WT_ERR(__backup_uri(session, cfg, &target_list, &log_only));