So you're saying that if a population developed a shell of some sort and that shell offered a survival advantage, you'd expect it to spread. If that spreading took millions of years it would appear to us to be instantaneous because the fossil record is so spotty.
Yep, pretty much what the scientists say. Along with possible changes in the water. Perhaps it started when some minerals precipitated out onto them, and this provided an advantage. The chemicals in/on the individuals that helped that were "selected for". The more efficient they were at collecting the calcium and then shaping it, the more advantage they had. Some were just mere tubes. Not perfect tubes, either. Wrinkly, bent tubes. If you were looking for a "transitional" fossil between "no shell" and "cool shells", there you have it, 540 million years ago.
And something like this could happen, start to finish, over millions of years, especially in the tiny, earliest "shellies". These tiny creatures likely multiplied quickly, even before having shells.
This is where you get into the math/theory of phase changes, which is crucial to punctuated equilibrium.
This link is to 8:12 of the video, which starts the relevant part , re: phase change
Point being, mathematicians propose that there are likely tipping points in punctuated equilibrium. These are the phase changes. And that, when a trait with a strong enough advantage spreads far and quickly enough (but not even , necessarily, through
most of the individuals), then the trait can very quickly come to dominate the species. Even globally.