Does anyone know of circumstances that might cause books that I have already downloaded and opened to read within my iOS library, then closed again, to spontaneously begin redownloading? This has happened to me several times now and it is becoming increasingly frustration-making

I have tried deleting the Kobo app, reinstalling it and then redownloading my library (at least the five most recent titles), but this not solved the problem.
I have also noticed that when these spontaneous redownloads occur, they seem to be get slower and slower: presumably because the app is effectively rewriting the book file.
Oddly, this problem only seems to occur on my iPad, not my iPhone. I have the 2012 iPad, running (naturally) the latest version of iOS and am of course also using the latest version of the Kobo app. I am using a wifi not 3G/ cellular connection.