AFAIK, TREE does not sort at all by default. It just lists directory entries in the order they appear on the disk. The behavior you're seeing is just the way that the underlying file system works.
On NTFS, the filesystem itself sorts directory entries by name, though not necessarily in the same order that DIR would.
On FAT, it's essentially random. Older items will tend appear first, but that isn't guaranteed. Deleting an older file can open up a slot that may be reused by a newer file.