Specifically, this is a consequence of mount propagation.
Modern systems, including anything which boots with
systemd, enable mount propagation on
/ and child mounts by default. It is recommended generally, for the benefit of mount namespaces.
This means that when you create a bind mount, by default any mount operations you make underneath the bind mount are also applied to the source directory. (More usefully, the propagation also works the other way round).
If you have the misfortune to not know about this and end up over-mounting
/bin (which I'm guessing is a symlink to
/usr/bin) with something else, you won't be able to access your normal commands. Ouch.
If you want to find out a bit more about the effect of mount propagation, you could start by skimming through
man mount. It references some other documents as well.