I've noticed Safari dies pretty often when memory usage gets too high (tell tale sign is the GUI becoming unresponsive or really slow to act on touch events). On bungie.net this is particularly a problem probably because of the large amount of Javascript being used and the actual use of the Javascript. For instance, modifying multiple DOM elements whenever the page is resized. While that's something which has no performance impact on a desktop browser, on a mobile device it's a resource-hog and blocks the rest of the UI controls - back, forward, etc... Since the Telerik controls use a large amount of Javascript as well, they don't seem to want to play nice either. There's also at least one instance of a postback where Safari will (at least for me) crash every time.
As for being signed out, that's probably the result of Safari deleting cookies (as well as any other cached items) set by bungie.net before it crashed.