I don't even know if that's a term or not, but it makes sense in my head.
Here's what I meant by existential drift - it's that moment in time when you're thinking about the meaning of life, or why you're here, or what's your purpose, or what's it all mean, and then you shift from curiosity and awe into despair, angst, and terror. It's that reeeeally slow shift; that gradual slope that you don't catch until you're speeding down the slide into the deep, dark recesses of your existentially-terrified mind.