How frequently do pre-tenured assistant professors switch institutions?
It does happen. But it's not that common. I've been on faculty search committees for a number of years now, and out of the hundreds of applications I've seen, it's very rare to see pre-tenure faculty from other institutions apply. If and when they do, it's because


they're worried about their tenure case
there's a two-body problem (this is in fact the most "reasonable" reason to move)
they have other reasons for wanting to move (location, opportunity)


But it's not a frequent thing. Because it's hard to do: clocks have to be negotiated, research programs have to be uprooted, students have to be moved (and let me not even start on the logistics of moving a family that might just be getting settled in the first place)