The CBSO doesn't normally use this arrangement, and neither does Klaus Mäkelä's own orchestra the Oslo Philharmonic, so this was presumably an experiment. The Vienna Philharmonic regularly uses this setup with the basses at the back.
I would say that the conductor wanted to highlight the double bass section, both aurally and visually. I have seen this only once before, with a Russian orchestra, and a big section of basses. The impact was immense.