TestComplete scripts occasionally open multiple instances of IE instead of using a single instance

My scripts run through a series of pages. Almost every time, this is completed within a single instance of IE, as expected. Just occasionally though, I find the script has opened multiple instances of IE and failed. Microsoft, say this is not something wrong with their browser (IE11) and ask me to check our links, which are not changing and are fine, so now I'm seeing if this is a TestComplete thing.