Don't make your group 1 'unlimited' (set it to 1) otherwise that will keep putting topics in the first column indefinitely.
'Groups' (that are not set 100% width) means that topics will stack vertically (and not bump into the next column) until the Number of Sections to group has been satisfied.
May not make a whole lot of sense but this method is a great time saver (compared to the last version of Board) when you want to rearrange or add in a section later on.