Not at all unusual.
Most birds don't care to be disturbed while on the nest,
and will start yelling like that which can set off the rest of them.
They can all go off like that when one announces an egg whether you're there or not.
I don't believe that they think you are a predator.
Honestly, they really don't seem too upset, I'm betting y'all spend a lot of time out there so they are used to you....otherwise that girl would have dashed out of the nest and raised an even a bigger ruckus.
It's good, IMO, to be around them a lot, tho I try not to take eggs from under a bird.
If you keep observing like in the vid, one day you will see an egg actually emerge.