Not all the time, I must admit - but when I'm blindly poking my phone or it's just grabbed out of my pocket, some part of the screen (that for me is the most likely part) makes the swipe prompt disappear and is replaced with the passcode entry.

Given that I have my N5 in a case, perhaps it's an inadvertent button press. Still, the UI should always provide swipe access to disable an alarm while the device is blaring at me.