According to the Epic Battle Player's Guide section, this would be considered a bug.
If you choose your 2-3 companions and time runs out a second or two becore you hit done, randomly selected companions will appear. The guide says if you don't pick anyone for that silohette thingy, the companion will be randomly selected, but the companions that you did choose did take up that spot, yet are still randomly selected if you don't press done. Its probably not a bug, but it should be in the Player's Guide.