Wooden panels start to shift
The short answer is the wood is shrinking as it loses moisture. Because you've used two fastening points across the width of a top board, one or more of those top boards may split as it continues to shrink.

Accommodating this sort of change in dimension (which continues to happen seasonally, even when the wood has fully "dried") is one of the classic challenges in constructing furniture from solid wood.

There is a lot of literature out there on this topic. Here's the first link returned from Google on "furniture accommodate wood shrinkage"
https://www.extension.purdue.edu/extmedia/fnr/fnr-163.pdf

Note that it's both shrinkage and expansion that cause problems.