Problems with spreads--and why do some page numbers have brackets, as [16]?

My page spreads have gone squirrely. I'm moving pages from one document to another. There are 16 pages in the document 1. When I move pages from document 2 to the end of document 1, the first (right-hand) imported page of document 2 doesn't form a spread with p. 16 of document 1. The new, imported page 17 is on a new line below it. The last page of document 1 has brackets around the page number in the Pages window, as in [16].  I can drag the page 17 thumbnail up to make a spread, which is now numbered with brackets [16-17]. If I try to add two pages after p. 17, they don't form a new spread below [16-17], but instead make a FOUR-PAGE SPREAD [16-17-18-19]. If I try dragging 18-19 off this monstrosity, they don't align properly, either in the Pages window or in the main document window, as per attached images.

Help! How can I restore the spreads? And what do those brackets mean???