In the case of updating to OS X 10.10.4, it appears to be some battle between the AppStore servers themselves and the AppStore app on your Mac.
Of course it is possible that Apple’s servers are simply slammed right now, but this is a gigantic company that can afford bandwidth. You’re just going along minding your own business, trying to update to the latest shiny release of Apple OS X (10.10.4 in this case), and alas, the only vehicle to do this is Apple’s AppStore. I hope Apple pours some of their riches into better Software QA some day, but if you are attempting to update to Mac OS X Yosemite 10.10.4 and your AppStore is crashing and hanging, here is how to fix it.
