ffmpeg tile command: ordering frames top to bottom rather than left to right
You can't change the behaviour of the tile filter, but you can adjust your inputs before and after to get the desired result.

ffmpeg -i input -vf hflip,transpose=cclock,tile=4x4,transpose=clock,hflip out.png


Note that the layout supplied to tile is transposed, so if you want the final result to be 6x4, tile should be given 4x6.