The ocaml package has been updated in opam-repository in PR12832. Since all other packages depend on it, when you next run opam update, opam is going to want to rebuild all the packages in your switches. The compiler itself will not be rebuilt as part of this, “just” packages.
The ocaml package is a virtual package which probes the configuration of the underlying OCaml compiler (either a system compiler, an opam-compiled OCaml, or an opam-compiled patched compiler).
The reason for the change is to fix the creation of switches for compilers which don’t support shared libraries. The problem was that a script in the ocaml package depended on the Unix library to probe the output of ocamlc -where.
Sorry for the CPU-hogging inconvenience this will cause - it’s a package we aim to leave alone most of the time!