Apple is suspiciously silent about this one, but it's a problem a lot of people seem to be encountering. I've had it happen more than a few times myself, but not with any regularity...
i've found other problems with it as well, for me it crashes constantly, pages render wrong, links become unclickable and unscrollable, sometimes needing only a reload, sometimes needing safari to be shut down (and the system rebooted in one case, it suddenly refused to relaunch after a crash...)
wish there was some way to go back. Looks like i might be making the switch back to a mozilla-clone before long, which i really don't want to do...
pnw