I've found a couple of related things to be very interesting.
The first is the pure fact of Information overload.
Sometimes there is so much activity and information coming at you that there is simply not enough computing capacity in your brain to process it all. At that point natural (and possibly reflexive) filters kick in. The best example I can think of is trying to keep an eye on your kid at a busy/crowded waterpark. Your brain prioritizes "watch the kid" to the exclusion of other stuff happening right near you.
Another thing I've noticed over time is that I'm often aware that a few seconds before I was unaware. That's always discomforting but again, your brain can only process so much at a time.
"The dogs may bark but the caravan moves on"