When you place a text file on a page that already has column guides applied, you can choose from various autoflow options to place the text within the columns automatically.
Without specifying autoflow, after you select the file you want to place, you see a loaded text icon on your page. If you click in a column, the text flows into that column and an overset text symbol shows, enabling you to click and reload the text icon.
If you hold down the Alt key (Windows users) or Option key (Mac users) when you click the loaded text icon in the first column, the text flows into the column and then automatically reloads so you can click and flow it into the next column.
If you hold down the Shift key when you click in the first column, the text automatically flows into the columns set up on the page and adds pages to place all of the text in the file.
If you hold down Shift+Alt (Windows users) or Shift-Option (Mac users) while you click, Fixed Page Autoflow is activated, and text is flowed onto that page only, without adding pages or columns. All unplaced text is overset.
You must have set your column guides in the Document Setup dialog so all added pages have the same column structure. If you applied columns to only the current page (through Layout, Margins and Columns), only the first page has the column structure you specified; all added pages have one column.