Microsoft’s patch tuesday 8/12/15 brought update KB3114409.
The patch was meant to fix issues with Outlook 2010 however it breaks Outlook and forces it into running in Safe mode and also prevents the preview pane from working unless you manually turn it on.
The fix is to uninstall the patch. Go into Programs & Features, View installed updates and search for the kb number. Double click it to uninstall.
Microsoft have pulled the update.