Failed AppStore download
Check your versions of Pages, Numbers &amp; Keynote &amp; make sure they're not updating in the background [look at Launchpad for the telltale thermometer] - mine updated to x.5.3 when I first tried updating to 10.10.4 &amp; I had to wait until they were finished before the OS update would run. 

If they are up to date, or at least not currently updating, then I'd get the combo updater &amp; use that instead, from - https://support.apple.com/kb/DL1820